Stop waiting for cookie season! This Thin Mint Sheet Cake is the ultimate year-round fix for your Girl Scout cookie cravings. We’re talking three layers of minty perfection: a crunchy cookie crust, a rich chocolate-mint cake, and a duo of silky chocolate ganache and creamy mint frosting. Whether you use OG Thin Mints, Goodie Girl Gluten-Free, or Mint Oreos, this decadent sheet cake is a guaranteed crowd-pleaser for every mint-chocolate lover.
What You’ll Need
The secret to this cake is elevating a standard box mix with high-fat dairy and extra hits of mint. Here’s a breakdown of the key players:
The Dry Goods
- Chocolate Cake Box Mix: Use your favorite brand. We’re going to “doctor” this up to make it taste bakery-fresh.
- Unsweetened Cocoa Powder: Stick with natural cocoa powder here rather than Dutch-process to ensure the right rise and texture.
- Mint Cookies: You’ll need 2 boxes of Girl Scout Thin Mints.
- Pro-Tip: If it’s not cookie season, grab 3 boxes of Goodie Girl Gluten-Free Mint Cookies or a large pack of Mint Oreos.
- Edible Glitter (Optional): For a touch of magic, I love using Fancy Sprinkles edible gold glitter.
The Dairy & Fats (Room Temp is Key!)
- Buttermilk: This adds a beautiful tang and moisture. If you don’t have it, make a DIY sour milk by adding 2 tsp of vinegar or lemon juice to 1 cup of whole milk and letting it sit for 5 minutes.
- Sour Cream: The secret weapon for a dense, moist crumb. Ensure this is at room temperature before mixing.
- Unsalted Butter: Melted. Using unsalted allows you to control the salt levels perfectly.
- Avocado Oil: Cocoa powder can be drying; oil provides the necessary fat for a silky mouthfeel. Vegetable or canola oil also works great.
- Large Eggs: Bring these to room temperature so they incorporate easily without over-beating the batter.
The Flavor Boosters
- Mint Extract: You can also use peppermint extract for that classic cool finish.
- Pure Vanilla Extract: It rounds out the chocolate flavors. (I’m a big fan of the Costco brand for quality and value!)
Why Room Temperature?
- You’ll notice I mentioned “room temperature” several times. When your eggs, sour cream, and buttermilk aren’t cold, they emulsify much better into the fats. This results in a smoother batter and a more even bake.
What Ingredients You Need To Make Thin Mint Chocolate Buttercream
A great sheet cake is only as good as its frosting. Here is what you’ll need to create that creamy, minty finish:
The Creamy Base
- Unsalted Butter: Make sure it’s perfectly room temperature. I usually take mine out a few hours before baking.
- Pro-Tip: Your butter is ready when your finger leaves a slight dent when pressing it. If you’re in a rush, zap it in the microwave for 20 seconds—just be careful not to melt it!
- Powdered Sugar: Also known as confectioners’ sugar. Good news: You don’t even need to sift it for this recipe!
- Heavy Cream: This creates that luxurious, whipped texture. If you’re in a pinch, whole milk works too.
- Unsweetened Cocoa Powder: Stick with natural cocoa powder here to keep the flavors balanced.
Flavor & Crunch
- Mint Cookies: This is where that signature crunch comes in.
- Girl Scout Thin Mints or Goodie Girl: Use 16 cookies.
- Mint Oreos: Use 13 cookies.
- Mint & Vanilla Extract: A combo of both gives the frosting depth. (And yes, the Costco vanilla is a total staple!)
- Salt: A pinch of sea salt or pink Himalayan salt is the secret to making the chocolate and mint flavors truly pop.
Baker’s Quick Guide: Room Temp Butter
- If you’ve ever tried to whip cold butter, you know the struggle. Bringing your butter to room temperature ensures your frosting is silky-smooth and easy to spread over that delicate mint cookie crust.
What Ingredients You Need To Make Chocolate Ganache
A decadent sheet cake deserves a crown. This two-ingredient ganache is the “ribbon” that ties the mint and chocolate layers together.
- Milk Chocolate Chips: I highly recommend using Guittard milk chocolate chips. The quality of the chocolate really shines here, giving you a smooth, professional-grade melt.
- Heavy Cream: This is what turns your chocolate into a pourable, velvety dream.
- Quick Swap: If you’re fresh out of heavy cream, you can use whole milk in a pinch, though the ganache won’t be quite as thick or rich.
What Supplies You Need
The Baker’s Toolkit
To get those bakery-style results at home, here are the tools I recommend. Whether you’re a pro with a piping bag or just starting out, these will help you achieve that perfect “Thin Mint” look.
Essential Gear
- 9×13 Inch Baking Pan: Metal, glass, or even a disposable pan works perfectly for this sheet cake.
- My Favorite: I personally swear by Fat Daddio pans for their even heat distribution and sharp corners.
- Food Processor: You’ll need this to pulse those mint cookies into the perfect fine crumbs for your crust and frosting.
- Offset Icing Spatula: This is the secret to a perfectly smooth ganache layer. If you don’t have one, the back of a large spoon works in a pinch!
For the Finishing Touches
- Piping Bags & Tips: I love using a 1M piping tip (you can find these at Michaels) to add those classic, professional swirls to the top of the cake.
- Piping Bags: I usually grab mine in bulk from Amazon so I’m always ready to bake!
How To Make A Thin Mint Chocolate Sheet Cake
The Perfect Mint Cookie Crust
The base of this sheet cake is a crunchy, minty layer that sets the stage for the rich chocolate cake above. The best part? No pre-baking required!
Instructions:
Prep Your Space: Preheat your oven to 325°F. Generously grease your 9×13 inch cake pan (I love my Fat Daddio pan for this!).
Pulse to Perfection: In a food processor, blend your mint cookies until they reach a fine, sandy powder.
Mix it Up: Pour the cookie crumbs into a bowl, add the melted butter, and stir until the mixture looks like wet sand.
The “Cup Trick”: Transfer the mixture to your prepared pan. To get a truly professional, even layer, use the bottom of a flat measuring cup to press the crust firmly into every corner.
Cookie Substitution Guide:
If it isn’t Girl Scout season, don’t worry! You can swap the crust cookies easily:
Girl Scout Thin Mints: 1 box (approx. 9 oz or 32 cookies).
Mint Oreos: 10 oz (approx. 25 cookies).
Goodie Girl Gluten-Free Mint Cookies: 9 oz (you’ll likely need 2 boxes to hit the weight).
Baker’s Note:
Since we aren’t pre-baking this crust, the weight of the cake batter will help “set” it into the pan as it bakes. Just make sure you’ve pressed it down firmly so you get that satisfying crunch in every bite!
Whisk the Liquid Base
For a “doctored” cake mix to taste like it came from a high-end bakery, the secret is in the emulsion of the wet ingredients.
Instructions:
Combine the Dairy & Eggs: In a large mixing bowl, whisk together the room-temperature buttermilk, sour cream, and eggs.
Add the Aromatics: Pour in the oil, vanilla extract, and mint extract.
Whisk until Smooth: Continue whisking until the mixture is fully combined and smooth.
Why this works:
By whisking these ingredients before adding the dry mix, you ensure the mint and vanilla are evenly distributed. More importantly, the combination of buttermilk and sour cream creates a thick, creamy base that guarantees a fudgy, “from-scratch” texture once the cake is baked.
Incorporate the Chocolate Base
Now it’s time to turn those wet ingredients into a decadent batter. Adding extra cocoa powder here is the secret to getting that intense chocolate color and flavor that perfectly matches a Thin Mint.
Instructions:
Mix & Pour: Add your unsweetened natural cocoa powder and the chocolate cake box mix directly into your bowl of wet ingredients.
Whisk to Combine: Whisk the mixture thoroughly until the batter is smooth and no large lumps remain.
The Final Scrape: Use a rubber spatula to scrape the bottom and sides of the bowl to ensure every bit of dry mix is fully incorporated.
Why Natural Cocoa?
Since we’re using a box mix, natural cocoa powder reacts best with the leavening agents already in the mix. It provides that classic, sharp chocolate punch that balances out the cool mint perfectly.
Baker’s Tip for the Best Texture:
While you want to ensure everything is “thoroughly combined,” try not to over-whisk once the flour is in. A few small lumps are okay—over-mixing can lead to a tougher cake, and we want this sheet cake to be as moist and tender as possible!
Pour and Bake
It’s time to bring the crust and the batter together. This is where the kitchen starts to smell like a dream!
Instructions:
Fill the Pan: Pour your rich chocolate batter over the prepared mint cookie crust. Use your offset spatula to spread it evenly into the corners.
Into the Oven: Place the pan on the center rack and bake at 325°F for 27 to 30 minutes.
The Toothpick Test: Since every oven runs a little differently, start checking at the 27-minute mark. Insert a toothpick into the center of the cake—when it comes out clean or with just a few moist crumbs, it’s done!
Pro-Tip for the Perfect Bake:
Avoid opening the oven door too early! Opening it repeatedly can cause the temperature to drop and may affect the rise of your cake. Stick to the “light-on-and-peek” method until you’re ready to test with the toothpick.
The Cooling Period
Once that timer goes off, the hardest part begins: the wait! It is essential to let the cake cool completely in the pan before you even think about decorating.
Why the Wait?
The Crust: As the cake cools, the mint cookie crust fully sets, ensuring it won’t crumble when you slice into it later.
The Toppings: If you apply the chocolate ganache or the creamy mint frosting to a warm cake, they will melt right off! A room-temperature cake is the only way to get those clean, professional-looking layers.
Multi-Task Like a Pro:
While your cake is cooling on the counter, use this time to prep your decorations:
Whip the Frosting: Get that mint cookie frosting ready to go.
Prep the Ganache: Warm up your heavy cream and Guittard chocolate chips so the ganache is smooth and ready to pour the moment the cake is cold.
Baker’s Tip:
If you’re in a time crunch, you can move the pan to a wire cooling rack to help air circulate underneath. Just don’t rush the process—your patience will be rewarded with a perfect, stable slice!
Pro-Tip: The “Chill Factor” for a Crumb-Free Finish
If you want that perfectly smooth frosting layer without any pesky chocolate crumbs trailing through it, try my favorite trick:
Once the cake has reached room temperature, pop it into the refrigerator or freezer (covered) while you whip up your frosting and ganache.
Why this works: Chilling the cake “locks” the crumb in place. When the cake is cold and firm, your frosting will glide over the surface like a dream, rather than pulling at the delicate sponge. It’s the easiest way to get that professional, clean look every single time!
How To Make Thin Mint Chocolate Buttercream
The Creamy Mint Cookie Frosting
This isn’t your average buttercream. By folding in finely pulsed mint cookies, you get a beautiful speckled look and a texture that is absolutely addictive. This recipe makes about 4 cups—plenty for a thick layer and some decorative swirls!
Instructions:
Prep the Cookies: In a food processor, blend your mint cookies until they reach a fine, sandy powder.
How many? You’ll need 16 Girl Scout or Goodie Girl cookies, or 13 Mint Oreos.
Cream the Butter: Using a stand mixer (paddle or whisk attachment) or a hand-held mixer, beat the room-temperature butter on medium speed for about 2 minutes until it’s light and fluffy.
Add the Aromatics: With the mixer running on low, slowly pour in the vanilla extract, mint extract, heavy cream, and salt.
Build the Flavor: Gradually add the unsweetened cocoa powder, powdered sugar, and your prepared mint cookie powder.
The Final Whip: Increase the speed to high and beat for 3 full minutes. This incorporates air and makes the frosting incredibly smooth and spreadable.
Dial in the Consistency:
Every kitchen environment is different! Use these quick fixes to get your frosting exactly where you want it:
Too thin? Add a little more powdered sugar, one tablespoon at a time.
Too thick? Splash in a tiny bit more heavy cream.
Too sweet? Add a tiny pinch of salt to balance the sugar.
Baker’s Note on Piping:
Because of the cookie crumbs, make sure your powder is as fine as possible. If you plan on using your 1M piping tip later, a fine powder ensures the frosting flows perfectly through the tip without any clogs!
How To Make Chocolate Ganache
The Glossy Chocolate Ganache
This simple two-ingredient glaze adds a professional, high-end finish to your sheet cake. It’s rich, silky, and the perfect “glue” for your final cookie crumbles.
Instructions:
Combine: Place your Guittard milk chocolate chips and the heavy cream into a medium microwave-safe bowl.
Heat: Microwave the mixture for 30 seconds.
Rest & Stir: Let the bowl sit for about a minute to allow the warm cream to soften the chocolate, then whisk gently starting from the center until the mixture is completely smooth and glossy.
Baker’s Tip:
If you still see a few stubborn chocolate chips after stirring, microwave it for another 10 seconds, but be careful! Chocolate can burn easily. You want it just warm enough to melt into a pourable consistency.
Perfecting the Melt
The Incremental Method: After the initial 30 seconds, give the mixture a good stir. If you still see unmelted chips, return the bowl to the microwave and heat in 15-second increments.
Stir, Stir, Stir: Be sure to stir well after each heat burst. The residual heat from the cream often does more work than the microwave itself!
The Goal: Stop heating as soon as the chocolate is completely melted and the ganache looks like a smooth, dark ribbon.
Set Aside: Let the ganache sit for a few minutes at room temperature to thicken slightly before you pour it over your chilled cake.
How To Decorate The Cake
The Frosting & The Flash-Freeze
Now it’s time to build those layers! Using a cold cake and a quick trip to the freezer is the secret to getting those distinct, beautiful layers you see in professional bakeries.
Instructions:
Spread the Base: Scoop about 2 cups of your mint chocolate frosting onto the center of your chilled cake.
Smooth it Out: Using your offset icing spatula, spread the frosting evenly across the top, reaching all the way to the edges of the pan.
The 5-Minute Flash-Freeze: Place the frosted cake into the freezer for exactly 5 minutes.
Why the Freezer?
This quick chill “sets” the top of the frosting. When you pour the warm chocolate ganache over it in the next step, the cold surface will help the ganache set almost instantly. This prevents the two layers from swirling together, keeping your frosting crisp and your ganache glossy!
The Ganache Glaze
Now for that glossy, professional finish! Adding the ganache over the “set” frosting creates a beautiful contrast in both texture and flavor.
Instructions:
The Pour: Pour your smooth, slightly cooled chocolate ganache over the center of the frosted cake.
Smooth & Level: Use your offset spatula to spread the ganache evenly over the top. Since your frosting was chilled, you’ll notice the ganache starts to thicken and hold its place perfectly.
The Final Chill: Place the cake back into the freezer for another 5 minutes.
Why do this?
This second chill “locks in” the ganache. It creates a stable, non-sticky surface so that when you pipe your 1M stars or swirls on top, they sit perfectly on the surface rather than sliding or sinking into the chocolate.
The Grand Finale:
Your cake now has a crunchy crust, a moist sponge, a creamy frosting, and a silky ganache. It’s officially ready for the “wow factor” decorations!
The Finishing Flourish
With your cake chilled and your ganache set, it’s time to add the decorative borders. This is where those 4 cups of frosting really come in handy!
Instructions:
Prep the Bag: Fit your piping bag with the 1M tip and fill it with your remaining mint chocolate buttercream.
The “Star” Technique: Hold the bag vertically and pipe even swirls or stars around the entire perimeter of the 9×13 pan.
The Design: Space them evenly to create a frame for the glossy ganache center. The contrast of the textured frosting against the smooth chocolate glaze is what makes this cake a real showstopper.
The Signature Grid & Toppings
To finish the cake, we’re going to add a bit of geometric flair and plenty of cookie crunch. This ensures every single slice gets a “crown” of frosting and a whole cookie!
Instructions:
The Grid Design: Using that same 1M piping tip, pipe three even vertical lines across the cake, connecting the top and bottom borders. This creates a beautiful “panel” effect.
Baker’s Note: You’ll likely need to refill your piping bag one last time to finish these lines with full, consistent pressure.
The Cookie Crown: Place 12 to 16 whole mint cookies onto the frosting swirls or along your piped lines. This acts as a guide for cutting—giving each guest their own signature cookie.
The Final Flourish: Edible Gold
If you’re using the Fancy Sprinkles gold glitter, now is the time to give the cake a light dusting. The gold catches the light against the dark chocolate ganache and makes the whole dessert feel like a celebration.
Slice, Serve, and Store
You’ve done the hard work—now it’s time to enjoy the payoff.
Serving: For the cleanest slices, use a sharp knife dipped in warm water and wiped dry between each cut. The chilled ganache and cookie crust will give you those perfect, defined layers.
Storage: If you have leftovers (though they never last long!), cover the pan tightly with plastic wrap or transfer slices to an airtight container.
Shelf Life: Store the cake in the refrigerator for up to 5 days. The flavors actually deepen overnight, making it a fantastic “make-ahead” dessert!
Tips for the Perfect Thin Mint Sheet Cake
Before you preheat your oven, here are my top “Bakers” secrets to ensuring your sheet cake comes out bakery-perfect every single time:
- Avoid the Over-Mix: Once you add your cake mix and cocoa powder, whisk just until combined. Over-mixing can over-develop the gluten, which turns a moist, fudgy cake into one that is tough, dry, and dense.
- The Room Temp Rule: For both the cake and the frosting, ensure your buttermilk, sour cream, eggs, and butter are all at room temperature. This allows the batter to emulsify effortlessly, giving you a smooth, even crumb.
- The “Dent” Test for Butter: Cold butter is the enemy of silky frosting! Your butter should be soft enough that a light press of your finger leaves a dent. If it’s too cold, you’ll end up with lumpy buttercream.
- Patience is a Virtue: Never frost a warm cake! If the cake hasn’t cooled completely, your beautiful mint buttercream will melt right into the sponge.
- Chill for a Clean Finish: The longer you chill the cake before frosting, the better the layers will adhere. A cold cake “locks” the crumbs in place, ensuring your frosting stays pristine and easy to spread.
Frequently Asked Questions
Can I make this cake in advance?
Yes! This is a great “make-ahead” dessert. You can bake the cake and prep the ganache and buttercream a few days in advance. Store them separately and tightly wrapped in the fridge. To Assemble: Pull the buttercream out a few hours early to reach a spreadable consistency. I recommend giving it a quick whip in your mixer before adding it to your piping bags. The Ganache: Gently zap the ganache in the microwave for 15 to 30 seconds to make it pourable again. Fully Assembled: You can also store the entirely finished cake in the fridge for up to 5 days.
Can I turn this recipe into a 2 layer cake?
Absolutely! If you want a more “towering” look, follow these steps: Prep the Pans: Grease two 8-inch or 9-inch round cake pans. Line the bottoms with parchment paper rounds and grease the paper to ensure a seamless release. Divide & Bake: Split the batter evenly between the two pans. Timing: Bake at 325°F for 27 to 30 minutes. Since round pans can bake differently than a 9×13, keep a close eye on them and use the toothpick test starting at 25 minutes.
Can I turn this recipe into a bundt cake?
Yes! This rich, moist batter works beautifully in a bundt pan. Prep the Pan: Generously grease every nook and cranny of your bundt pan with butter or a professional baking spray to ensure all those beautiful details come out clean. The Bake: Bake at 325°F. Because bundt cakes are thicker and deeper, they generally take longer to bake—start checking with a toothpick at the 45-minute mark. It’s done when the toothpick comes out clean or with just a few moist crumbs. Cooling: Let the cake cool in the pan for about 10 minutes before inverting it onto a wire rack. This helps the structure set so it doesn’t break when you flip it!
Can I turn this recipe into cupcakes?
Yes! This batter makes incredible cupcakes that stay moist for days. Prep: Line your cupcake pan with liners. I recommend using a baking scoop to fill each liner about 2/3 full to ensure they bake up evenly without overflowing. The Bake: Increase your oven temperature to 350°F. Bake for 14 to 19 minutes, or until a toothpick comes out clean. This recipe yields approximately 22 regular-sized cupcakes. Pro Decorating Tip: For a “bakery-style” surprise, core the center of the cooled cupcakes and fill them with the chocolate ganache. Top them with a generous swirl of the mint chocolate buttercream and a whole mint cookie for the perfect finish!
Can I make this with store bought frosting?
Yes! If you’re short on time, you can definitely use your favorite store-bought chocolate frosting as a base. How much? I suggest having 2 to 4 containers on hand (depending on how thick you like your frosting layer and how much piping you plan to do). The “Upgrade”: To give it that bakery-style flavor and texture, simply follow Step 6 of the instructions. Folding in those finely crushed mint cookies and giving it a good whip with your mixer will make it taste completely homemade!
I don’t like cake box mix, can I use a homemade sheet cake recipe?
Yes! If you prefer a scratch-made base, you can absolutely use my homemade chocolate sheet cake recipe. The process remains almost identical: The Base: Prepare the mint cookie crust in your 9×13 pan as directed in Step 1. The Batter: Pour your homemade chocolate batter directly over the unbaked crust. The Bake: Follow the baking temperature and time specified in your scratch recipe (usually around 325°F or 350°F). Just keep an eye on it—the cookie crust won’t affect the bake time of the cake itself. The Toppings: Once cooled, proceed with the mint buttercream and ganache layers as usual!
Do I have to decorate it they way you did?
Of course not! If the piping or the grid design feels a bit too technical, or if it’s just not your style—don’t sweat it. You don’t have to follow my design to a tee to have a delicious, showstopping cake. Whether you want to skip the piping bags and just do a rustic swoop of frosting, or try a completely different topping altogether, feel free to get creative. You do you, boo! The most important part is that you enjoy the process (and the cake).
How do I store leftover frosting?
Don’t let a single drop of that mint chocolate buttercream go to waste! Whether you made a double batch or just have a little left over from your piping bag, here is how to keep it fresh: In the Fridge: Store in a tightly sealed airtight container for up to seven days. Pro-Tip: Keep the container away from pungent foods like seafood or onions—buttercream acts like a sponge for smells, and nobody wants a “shrimp-flavored” mint cake! In the Freezer: You can freeze buttercream for up to 3 months in a freezer-safe bag or container. To Reuse: The day before you need it, thaw it in the refrigerator overnight. On the day of decorating, bring it to room temperature and give it a quick re-whip in the mixer to restore that light, fluffy texture before filling your piping bags.
How do I store cake leftovers?
If you happen to have any cake left over (a rare occurrence!), here is how to keep it tasting just as fresh as the day you baked it: Room Temperature: Cover the cake tightly and store it on the counter for up to 3 days. Refrigeration: For longer storage, keep it in the fridge for up to 5 days. This is actually my preferred method for this specific cake, as it keeps the ganache and frosting nice and firm. Freezing (The Long Haul): You can freeze this cake—frosted or unfrosted—for up to 2–3 months. Pro-Tip: To prevent freezer burn, wrap the cake (or individual slices) thoroughly in a layer of plastic wrap followed by a layer of aluminum foil. To Serve: Thaw the cake overnight in the refrigerator. For the best texture, let it sit on the counter for about 30–60 minutes to bring it to room temperature before serving.
Make sure you post your photos on Instagram and tag me in them so I can see your creations from my blog #eazypeazylemonsqueezee
Thin Mint Cookie Sheet Cake
- March 5, 2024
- 12
- 4 hr
- Count the memories not the calories! Cals/Serving
- Print this
Ingredients
- Thin Mint Chocolate Cake:
- 32 (1 box) Thin Mint Cookies (or 25 (10 ounces) Oreo Mint Cookies or 9 ounces Goodie Girl Gluten Free Mint Cookies), pulsed to a powder in a food processor
- 4 tablespoons unsalted butter, melted
- 2/3 cup buttermilk, at room temperature
- 2/3 cup sour cream, at room temperature
- 3 eggs, at room temperature
- 1/3 cup oil (avocado oil, vegetable oil, canola oil or melted coconut oil)
- 1 Tablespoon pure vanilla extract
- 1/2 teaspoon mint extract
- 1/4 unsweetened natural cocoa powder
- 1 box chocolate cake mix
- Thin Mint Chocolate Buttercream:
- 1 cup (2 sticks) unsalted butter, at room temperature
- 1/4 teaspoon salt
- 1 Tablespoon pure vanilla extract
- 1/2 to 1 teaspoon mint extract
- 1 to 2 cups heavy cream
- 2/3 cup unsweetened natural cocoa powder
- 7 & 1/2 cups (2lbs) powdered sugar
- 16 Thin Mint Cookies (or 16 Goodie Girl Gluten Free Mint Cookies or 13 Oreo Mint Cookies), pulsed to a powder in a food processor
- Chocolate Ganache:1 cup chocolate chips
- 1/2 cup heavy cream
- Toppings:
- 12-16 Thin Mint Cookies (or Goodie Girl Gluten Free Mint Cookies or Oreo Mint Cookies)
- Edible gold glitter (optional)
- Supplies:
- 9x13 inch sheet pan
- Offset Icing Spatula
- Pipping bags
- Pipping tips 1M
- Food Processor
Directions
- Step 1
- PREP CAKE: Preheat the oven to 325°F. Generously grease a 9×13 inch cake pan.
- Step 2
- MAKE COOKIE CRUST: In a food processor, blend up the mint cookies until a fine powder forms. Add in the melted butter and stir until combined. (Note: You’ll need 1 box (9 ounces or 32 cookies) of Girl Scout Thin Mint Cookies. If you can’t get your hands on Girl Scout Cookies you can substitute with Oreo Mint Cookies (10 ounces or 25 cookies) or Goodie Girl Gluten Free Mint Cookies (9 ounces so you’ll need at least 2 boxes)). Press the crust in the prepared baking pan and press firmly in an even layer with the bottom of a cup. Pre-baking is not required.
- Step 3
- WET INGREDIENTS: In a large bowl, whisk together the buttermilk, sour cream, eggs, oil, vanilla, and mint extract until combined.
- Step 4
- COMBINE: Mix in the unsweetened cocoa powder and cake box mix. Whisk until throughly combined.
- Step 5
- BAKE: Pour the batter into the prepared pan. Bake for 27 to 30 minutes or until a toothpick inserted in the center comes out clean. Baking times vary, so keep an eye on yours. Remove from the oven and allow the cake to cool in the pan. Cool completely before decorating. As the cake is cooling, you can make the frosting and chocolate ganache to decorate the cake. (Tip: I like placing my cakes covered in the refrigerator or freezer while I make my frostings and/or toppings. I do this so that when I spread the frosting on the cold cake, the cake doesn’t pull away and I have no issues with crumbs in the frosting).
- Step 6
- FOR THE THIN MINT CHOCOLATE FROSTING: In a food processor, blend up the Mint cookies until a fine powder forms. (Note: If using Girl Scout Thin Mint Cookies or Goodie Girl Gluten Free Mint Cookies you’ll need about 16 cookies and if you’re using Oreo Mint Cookies you’ll need about 13 cookies). Using a hand-held mixer or stand mixer fitted with a paddle or whisk attachment, beat the butter on medium speed until light and fluffy, about 2 minutes. Add vanilla extract, mint extract, heavy cream, and salt with the mixer running on low. Slowly add the unsweetened cocoa powder, powdered sugar, and the mint cookie powder. Increase to high speed and beat for 3 full minutes. You can add more powdered sugar if frosting is too thin, more cream if the frosting is too thick, or a little more of salt if the frosting is too sweet. Makes about 4 cups of frosting.
- Step 7
- FOR THE CHOCOLATE GANACHE: Place chocolate chips and heavy cream in a microwave-safe bowl and microwave for 30 seconds. Stir well, and then return to microwave and heat in 15-second increments, stirring well in between, until chocolate is completely melted and smooth. Set aside.
- Step 8
- DECORATE THE CAKE: Evenly spread the chocolate buttercream over the top of the cake using an icing spatula. I used about 2 cups of frosting. Place the cake in the freezer for 5 minutes. I like doing this so that when you pour the warm chocolate over the frosting it will set quicker. Next, evenly spread the chocolate ganache over the frosting using an icing spatula. Place the cake in the freezer again for another 5 minutes. Using a pipping bag filled with mint chocolate buttercream and a 1M pipping tip, pipe the design (see above) evenly all the way around the pan. With the same 1M pipping tip pipe 3 even vertical lines. You might need to refill your bag with more frosting. Lastly you can place the rest of your mint cookies on top of the cake (you can add 12 to 16). You can also chop up some mint cookies and sprinkle them evenly over the top of the cake. If you’d like to take your decorating up a notch you can add gold edible glitter like it did. Slice and serve.
- Step 9
- Cover leftovers tightly and store in the refrigerator for up to 5 days.
