Bake cake according to package directions for a 9x13 inch pan; cool on wire rack for 5 minutes. Poke holes all over the cake with a toothpick. Pour the sweetened condensed milk all over the cake and let it soak in. Then spread the caramel topping over the cake. Cover and refrigerate until cooled completely. Top with whipped topping. Sprinkle the chopped toffee bars over the top. Refrigerate until ready to serve.