This cake captures everything you love about Monster Cookies—creamy peanut butter, soft oatmeal, mini chocolate chips, and mini M&M’s—transformed into a vibrant, irresistible layer cake that will have you craving the next slice the moment you finish the first.
Bright, cheerful, and playful, this cake is a love letter to anyone who remembers sneaking cookie dough as a kid. Building the flavors of classic Monster Cookies into a cake was so much fun—the result is nostalgic, indulgent, and seriously crowd-pleasing.
“Monster” desserts usually combine oats, peanut butter, and mix-ins like chocolate and candies. I wanted this cake to echo those familiar flavors, so it includes oatmeal, peanut butter, mini M&M’s, chocolate chips, and even eggless cookie dough for extra nostalgia.
If you’ve seen my other cakes, you’ll know I often include cookie dough—like in my Funfetti Cookie Dough Cake. The cookie dough in this recipe is eggless, making it safe to eat as-is; if you prefer, you can pre-bake the flour for extra caution. For this cake I chose to form large cookie dough scoops as a playful topper rather than sandwiching the dough between layers, though instructions for using it as a filling are included.
To incorporate oatmeal without risking odd texture from raw oats, I cooked old-fashioned oats on the stovetop and then folded the cooled oatmeal into the batter. The cooked oats blend into the batter beautifully and give the cake a softer, almost tender crumb that complements the Monster Cookie theme without overpowering the cake’s structure.
For peanut butter, I recommend using classic creamy Skippy for the familiar, reliable flavor it brings to both the cake and the buttercream. This is a dessert you’ll want to enjoy without worrying about being “healthy”—save your all-natural peanut butter for sandwiches and use the jar you love for baking.
The peanut butter buttercream in this recipe is a fan favorite and spreads easily. It’s rich, smooth, and perfect between layers and all over the cake. Use the same Skippy guideline here for best results.
When folding in mini M&M’s, be gentle—these candies can bleed color if overmixed. Stir them in at the last moment just until distributed so they keep their vibrant appearance.
The chocolate ganache adds both visual appeal and extra chocolate flavor. I used Wilton cocoa candy melts, which have a different melt ratio than traditional chocolate chips. If you choose to use chocolate chips, you may need to adjust the heavy cream amount to reach your desired ganache consistency—add more chips if the ganache is too thin or more cream if it’s too thick.
Thanks to everyone who asked about this cake after the Instagram tease—this one was worth the wait. Dive in and enjoy every colorful, peanut-buttery, cookie-dough studded bite.
Tag me on Instagram @bakingwithblondie if you make this—I love seeing your creations.
xo, Mandy
Here’s a little video showing the process:
- MONSTER COOKIE CAKE
- 1/2 cup old-fashioned oats
- 1 1/2 cups water
- 3 whole eggs, room temperature
- 1/2 cup sour cream
- 1/4 cup Skippy peanut butter
- 1/3 cup vegetable oil
- 3/4 cup buttermilk
- 1 tablespoon vanilla
- 1/4 cup mini chocolate chips
- 1/3 cup mini M&M’s
- 1 box “Butter Golden” cake mix (Duncan Hines recommended)
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- EGGLESS MONSTER COOKIE DOUGH
- 1/2 cup butter, slightly softened
- 1/2 cup granulated sugar
- 1/2 cup packed light brown sugar
- 2 tablespoons milk
- 2 tablespoons vanilla extract
- 1/4 teaspoon salt
- 1 cup all-purpose flour (pre-bake if desired)
- 3 tablespoons oatmeal
- 1/2 cup mini chocolate chips
- 1/2 cup mini M&M’s
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- PEANUT BUTTER BUTTERCREAM
- 2 sticks unsalted butter (1 cup)
- 2/3 cup peanut butter (Skippy recommended)
- 1 pinch salt
- 1 tablespoon vanilla
- 1/4 cup heavy cream
- 5–6 cups sifted powdered sugar
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- GANACHE DRIP
- 1 cup chocolate cocoa candy melts
- 1/4 cup heavy cream
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- GARNISHES
- Mini M&Ms
- Mini chocolate chips
- FOR THE CAKE:
- Preheat the oven to 325°F (convection). Prepare three 6″ cake pans with shortening and flour; set aside.
- In a small saucepan, combine oats and water and simmer over medium heat until thick and cooked through, about 5–10 minutes. Transfer to a bowl and cool completely.
- In a medium bowl, whisk together cooled oatmeal, eggs, sour cream, peanut butter, vegetable oil, buttermilk, and vanilla until combined.
- Sift in the cake mix and gently fold to combine without overmixing. Fold in mini chocolate chips, then very gently fold in mini M&M’s to avoid color bleed.
- Divide batter evenly between prepared pans and bake 30–35 minutes. Cool in pans 2 minutes, then turn out onto a wire rack to cool to room temperature. Wrap and freeze for a few hours or overnight—cold cakes are easier to frost.
- FOR THE EGGLESS COOKIE DOUGH:
- In a stand mixer fitted with the paddle, cream butter and both sugars until light. Add milk, vanilla, and salt, scraping down the bowl as needed.
- Mix in flour (pre-baked if desired), raw oatmeal, chocolate chips, and mini M&M’s until combined. Scoop the dough into large balls, place on a tray, cover, and chill. (Optional: press into pans and freeze briefly if using as a layer.)
- FOR THE BUTTERCREAM:
- Beat butter and peanut butter together until smooth. Add salt and heavy cream, then gradually add powdered sugar 1/2 cup at a time. Beat on high for 2 minutes until light and fluffy; adjust cream for desired consistency.
- FOR THE GANACHE:
- Microwave candy melts and heavy cream in a small bowl for 30 seconds, stir until smooth. Transfer to a squeeze bottle for drips.
- ASSEMBLY:
- Place the first cake round on a cardboard round with a dot of buttercream to secure. Spread a layer of buttercream, stack the next layer, and repeat with the final layer.
- Crumb coat the cake and freeze briefly. Apply the final buttercream layer, press mini chips and candies into the bottom rim, and chill briefly to set.
- Add the ganache drip, chill briefly, then top with cookie dough scoops and extra candies and chips.
- Serve and enjoy.