1. Bake cake according to package directions for a 9x13 inch pan; cool for 5 minutes. Make slits across the top of the cake, making sure not to go through to the bottom.
2. In a saucepan over low heat, combine sweetened condensed milk and caramel topping, stirring until smooth and blended. Slowly pour the warm topping mixture over the warm cake, letting it sink into the slits; sprinkle the crushed chocolate toffee bars liberally across the entire cake while still warm.
3. Let cake cool in pan completely. Top each slice with whipped topping, more chocolate toffee bar chunks, and swirls of caramel topping, if desired. Refrigerate 20 minutes. Serve from the pan.