To assemble the cake, place one of the cake layers on a cake board or serving platter. Spread an even layer of the frosting over the top of the cake. Top with a second cake layer and another layer of the frosting. Place the final cake layer on top. Frost the top and sides of the cake using a thin crumb coat (here's a quick video instruction from Martha Stewart). Place in the refrigerator for 30 minutes to set. Reserve about 1 cup of the remaining frosting in a pastry bag fitted with a decorative tip (I used a Wilton #21). Once the crumb cake is set, frost the top and sides of the cake again using an offset spatula for a smooth finish. Use the reserved frosting to pipe a border around the base of the cake (again, here's a video - shells are 1 minute into the video). Pipe swirls evenly around the top border of the cake - 16 swirls for 16 slices. Finish by garnishing with halved peanut butter cups. Refrigerate the cake until ready to serve.
To make the cake in two 9-inch pans, reduce the amount of batter by one third.
To make the cake in three 9-inch pans, increase the amount of batter by one third.
The baking times for alternate sizes may differ, so use the toothpick test to monitor doneness.
Source: Everyday Annie, adapted from Baking Illustrated