Add Frosting: Once ready to finish, microwave the tub of chocolate frosting for about 15 seconds to soften it (be sure to remove the lid and foil seal first). Stir the frosting until it’s easily spreadable. Remove the plastic wrap from the chilled cake and evenly spread the chocolate frosting over the top layer of graham crackers.
Chill Overnight: Cover the dish with plastic wrap again and return it to the fridge. Let it chill overnight to allow the graham crackers to fully soften.
Serve: When ready, slice the cake into squares and serve cold.