1. Make the cake layers: Preheat oven to 325°. Prepare cake batter according to package directions. Pour into 3 greased and floured 8-inch round cakepans. Bake at 325° for 20 to 25 minutes or until a wooden pick inserted in center comes out clean. Remove cake layers from the oven using oven mitts. Cool in pans on wire racks 10 minutes. Remove cake layers from pans, and cool completely on wire racks.
2. Prepare the frosting: Beat cream cheese, powdered sugar, and granulated sugar in a big bowl at medium speed with an electric mixer until sugar dissolves and mixture is creamy. Fold in whipped topping.
3. Frost and decorate the cake: Spread frosting between layers and on top and sides of cake. Cut candy bars into fun shapes, and press into frosting on top and sides of cake. Cover and chill at least 30 minutes or up to 1 day in advance.
Note: We decorated our cake using Snickers, Nestlé Crunch, and Kit Kat candy bars.