1. Bake cake according to directions for a 9x13 pan; cool on wire rack 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 sweented condensed milk and caramel topping, stirring until smoth 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 and serve right from the pan.