Preheat oven to 350 degrees. Coat two 9-inch round cake pans with cooking spray. At low speed beat mix, mayonnaise, eggs and 1 cup water 30 seconds. Increase speed to high; beat 2 minutes. Stir in chips.
Divide batter between pans. Sprinkle 1/4 cup toffee in each pan. Lightly spread toffee into batter. Bake 30 minutes or until toothpick inserted into centers comes out clean. Cool on racks 20 minutes. Remove from pans; cool completely on racks.
Transfer 1 cup frosting to pastry bag fitted with large star tip. Place 1 cake layer on serving plate; spread with 1 cup frosting. Top with remaining cake layer. Spread top and side with remaining frosting. Pipe lattice onto cake; garnish with toffee.
Note: You can also bake in a 13x9-inch pan for 30-35 minutes instead of round pans. Use 1 can frosting.