In a large zip-top bag, put in 3 1/2 cups of the Nilla wafers. Seal the bag fully to allow no air or contents out. Gently crush the Nilla wafers with the bottom of the frying pan or a rolling pin making coarse crumbs. Peel 2 medium ripe bananas and slice them thinly.
The cakes will have cooled by now, remove them from the pans. Trim each cake off using a serrated knife. Place one layer of cake with the cut side up onto a cake stand or serving platter. Use one cup of frosting on the one-layer cake and smoothen it into a thin layer using an offset spatula. Top it with sliced bananas and 1 cup of crushed Nilla wafers.
Top with the final cake layer. Using the rest of the frosting, cover the top and sides of the cake. Cover the edges of the cake with the remaining crushed Nilla Wafers by pressing a handful of crushed cookies against one side (work your way up from the bottom to top) so they adhere to the frosting. Any pieces that don't stick may be pushed back into the frosting until all sides are completely coated.
Snip off one corner of the bag of frosting. Decorate the top of the cake with tiny decorative mounds of frosting. Between each mound of icing, insert a whole Nilla Wafer.
The baked cake layers may be tightly wrapped in plastic and frozen for 1 day. The frosting can be kept for up to 2 days if stored in an airtight container. Allow it to come to room temperature before beating again.
Store the leftovers in the refrigerator loosely covered in plastic wrap for up to 3 days.