Make the chocolate cakes:
- Preheat the oven to 350F. Prepare the homemade chocolate cake mix recipe as directed in a large bowl and set aside.
- In a medium bowl, combine the remaining ingredients. Add the wet ingredients to the dry and stir until JUST combined. Do not over stir the batter. A few lumps are fine.
- Butter and flour two 8″x 2″cake pans: divide the batter evenly between the pans.
- Bake for 20-25 minutes until a toothpick comes out clean when inserted in the middle, adding additional time as necessary. Allow to cool completely before icing and/or decorating.
Make the peanut butter frosting:
- In the bowl of a stand mixer, or a large bowl with a hand mixer, use the flat paddle beat the butter on medium high until it is very, very smooth. Pause the mixer every minute or so to scrape down the sides.
- When the butter is very smooth, add the peanut butter and vanilla. beat on medium high until the butter and peanut butter are thoroughly incorporated. You may have to pause the mixer to scrape down the sides a couple times.
- Turn the mixer to medium-low and add the powdered sugar, ½ cup at a time, letting each measure incorporate into the mixture before adding the next half cup.
- When you’ve added 3 cups powdered sugar, taste the frosting and if desired, add the remaining ¼ cup powdered sugar and mix well. The frosting will look a bit grainy at this point. That’s normal.
- Remove the paddle blade, scrape the frosting from the blade into the bowl, and secure the whisk attachment. Turn the mixer on high and add the heavy whipping cream 1 Tbsp at a time, letting the mixer incorporate each teaspoon for about 30 seconds before adding another.
- When you’ve add all the heavy whipping cream, let the mixer whip for another 30 seconds and then turn it off. the frosting will lighten up in color and become incredibly fluffy. use on your cakes, cupcakes, or fillings as desired!
Assemble the Chocolate Peanut Butter Cup Cake:
- Once your cakes are cooled completely, and your peanut butter frosting is made, chop about 10 peanut butter cups into small pieces and set aside.
- Place the first completely cooled cake layer on a cake stand. Spread the peanut butter frosting on top and then top with half of the chopped peanut butter cups (these will form the middle layer).
- Place the second completely cooled cake layer on top of the frosted bottom layer and then spread the peanut butter frosting on the top and sides of both layers, covering the whole cake.
- Sprinkle the remaining chopped peanut butter cups over the top of the fully frosted cake and decorate with ½ slices of peanut butter cups (one for each slice you’ll cut).
Note: If you want to get fancy with your decorating, you can reserve ¾ cup of frosting for decorating the top and sides using a pastry bag and star tip to pipe frosting around the edges of the cake.