Once the cakes are completely cool, cut each of the three cakes into four equal size mini circles using a 3-inch diameter biscuit cutter. This should yield 12 cake circles to make your four 3-tier mini cakes or you can just make a normal cake and skip this step if you'd like.
Place four bottom layers on cake plate. Using an offset spatula, smooth about a 1/4 cup of frosting over the top of each cake bottom. Add four second layer cakes on top of frosting then repeat with another 1/4 cup of frosting on each of the four second layers. Add the top layer to each cake. Use the spatula to smooth the frosting around the outside of each mini cake. Refrigerate for 20 minutes to help solidify the cake layers.
Remove cakes from fridge. Spoon a large scoop of frosting on the top layer of each mini cake (about 1/2 cup each) and smooth the frosting over the top and sides of each cake. Add more frosting as needed to completely cover the cake. Smooth frosting into an even layer on the tops and sides. p.s. I suck at frosting cakes... hopefully yours won't have cake peaking though lol.
Return cake to refrigerator for another 20 minutes to chill the frosting. Prepare the ganache during this time.
Once chilled, slowly pour the slightly-cooled ganache over top of each cake, starting in the middle of the cake. When the ganache begins to drip over the sides of the cake, stop pouring.
Top with chopped peanut butter cup pieces. Refrigerate for another 15 minutes. [optional garnish: cut 16 miniature peanut butter cups in half and use them to rim the bottom of each mini cake]
Devour!! Then become ill from eating too much sugar.