Prepare cake mix according to package directions. Pour into prepared pan. Bake according to package directions. Cool cake in pan on a wire rack.
Using the handle of a wooden spoon, poke holes about 1 inch apart over top of cake. Pour sweetened condensed milk, then caramel topping over cake. Spread evenly with dessert topping. Before serving, sprinkle with chopped toffee bars. Store leftovers, covered, in the refrigerator for up to 24 hours. Makes 30 servings.