Prepare cake batter according to package directions. Pour into 3 floured, greased 8-inch round pans. Bake at 325°F for 20 to 25 minutes (or until toothpick comes out clean). Cool in pans 10 minutes. Remove and cool completely.
Beat cream cheese, powdered sugar, and granulated sugar at medium speed until creamy. Chop 8 candy bars finely. Fold cream cheese mixture and chopped candy into whipped topping.
Spread icing between layers and on top and sides of cake. Chop remaining candy bars, sprinkle 1/2 of chopped candy bars over cake. Press remaining chopped candy along bottom edge of cake.