Prepare the cake. Preheat the oven to 350°F. Butter the bottom and sides of two 8x3-inch round cake pans. Pour some cocoa powder in the pan and shake all over until evenly coated. Remove any excess by tapping the pans.
Sift all the dry ingredients into a large bowl, mix until well combined and set aside.
Whisk together all the wet ingredients in a large bowl until well combined. Slowly, mix the dry ingredients with the wet ingredients using a spatula until a smooth batter forms. Make sure you remove any lumps from the batter.
Evenly distribute the batter into the prepared pans. Place in the oven and bake for 40 - 45 minutes or until a toothpick comes out clean when you insert it in the center of the cake. Remove from the oven and allow cakes to cool in the pan for 10 minutes. Remove cakes from pans and allow them to cool over a cooling rack.
Prepare the frosting. In the bowl of a stand mixer, fitted with the paddle attachment, beat the butter and peanut butter at medium speed until smooth. About 2 minutes. Scrape de sides of the bowl as needed. With the mixer at low speed, gradually add the powder sugar and mix until well combined. Add vanilla and salt, mix. Scrape the sides of the bowl as needed. Add milk, beat at medium speed until smooth, and well combined.
Prepare the chocolate ganache. In a small saucepan, bring the cream to a boil over medium heat. Remove from heat. Place the chocolate in a small bowl and slowly add the cream. Whisk the mixture until smooth. Add the butter and whisk until incorporated.
Assemble the cake. Using a large serrated knife, cut both cakes horizontally as even as possible. Using both hands, place one layer of cake on a serving platter. Pipe frosting on top of the cake and add a few pieces of chopped peanut butter cups. Place another layer of cake and repeat the process. Finish the cake by icing top and sides with frosting. Decorate the cake with chocolate ganache, creating a dripping effect, frosting, and peanut butter cups of different sizes.