Grease and flour two 9-inch round cake pans. Prepare and bake cake according to package directions. Cool on wire rack. Carefully cut each layer horizontally to make 2 layers. In a bowl mix whipped topping with chocolate syrup and 6 crushed toffee bars (Heath or Skor). Place 1 layer of cake on serving plate and spread with chocolate mixture. Repeat with remaining 3 layers. Frost sides and top of cake. Sprinkle remaining crushed bar on top of cake.