Roughly chop the dark chocolate squares. Add half to the whipped topping and reserve the other half for decorating the cake.
Put the frosting in the fridge so it stays cold until you're ready to use it.
Let the cake cool completely on a wire rack before frosting, which typically takes about 1 hour. Once cooled, cut the cake in half like a hamburger bun.
Create the middle layer by placing a large dollop of frosting on the top of the bottom cake layer.
Place the top layer of cake on top of the frosting layer. Frost the cake with the remaining frosting. Decorate the top and sides of the cake with the remaining chocolate pieces. Serve as is or chilled.