Bake cake according to package directions for a 9x13 inch pan. While cake cools for just a couple minutes prepare topping. In a bowl, combine sweetened condensed milk and caramel topping, stirring until smooth and blended. Make holes across entire cake with end of wooden spoon making sure edges have holes or cracks as well. If using chocolate chips sprinkle on top. Slowly pour topping mixture over the top of the warm cake, letting it sink into the holes and covering edges; then sprinkle the crushed chocolate toffee bars liberally across the entire cake while still warm.  Let cake cool completely, then top with whipped topping. Decorate the top of the cake with some more chocolate toffee bar chunks and swirls of caramel topping. Refrigerate and serve right from the pan!