Preheat the Oven: Begin by preheating your oven to 350°F (175°C). Prepare two 9-inch round cake pans by greasing and flouring them to prevent sticking.
Mix the Dry Ingredients: In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt until combined.
Add Wet Ingredients: Add the eggs, milk, oil, and vanilla extract to the dry ingredients. Use a mixer or whisk to blend everything until smooth.
Incorporate Boiling Water: Gradually stir in the boiling water; this will help create a tender cake. Mix until the batter is uniform.
Fold in Peanut Butter Cups: Carefully fold in the chopped peanut butter cups into the batter, ensuring they're evenly distributed.
Bake: Divide the batter evenly into the prepared cake pans. Bake in the preheated oven for about 30-35 minutes or until a toothpick inserted in the center comes out clean.
Cool: Allow the cakes to cool in the pans for about 10 minutes before removing them to a wire rack to cool completely.
Prepare the Frosting: In a mixing bowl, beat the peanut butter, powdered sugar, and heavy cream until light and fluffy.
Assemble the Cake: Once the cakes are completely cool, place one layer on a plate, spread with frosting, and then top with the second layer. Frost the top and sides of the cake with the peanut butter frosting.
Decorate: Optionally, add additional peanut butter cups on the top for decoration. Slice and serve!
