Move oven rack to lowest position (remove other racks). Heat oven to 350°F.
In extra-large glass or metal bowl, beat cake mix and water with electric mixer on low speed 30 seconds; beat on medium speed 1 minute. Pour into ungreased 10-inch angel food (tube) cake pan. (Do not use fluted tube cake pan or 9-inch angel food pan or batter will overflow.)
Bake 37 to 47 minutes or until top is dark golden brown and cracks feel very dry and not sticky. Do not underbake. Immediately turn pan upside down onto glass bottle until cake is completely cool, about 2 hours. Run knife around edges of cake; remove from pan.
Crush or finely chop candy bars; reserve 1/3 cup. Fold remaining crushed candy into whipped topping.
Tear cake into about 1-inch pieces. In large bowl, mix cake pieces and whipped topping mixture. In ungreased 13x9-inch pan, lightly press cake mixture. Sprinkle with reserved crushed candy. Freeze dessert about 1 hour 30 minutes or until firm. Cut into squares or spoon into dessert dishes. Dust plates with cocoa powder. Store covered in freezer.