Follow directions on box for making cake. Easiest size is 9 x 13 for this cake.
After the cake is done, set it aside until its completely cooled (about 30 min - 1 hour).
Then, with the cake still in the pan, take the end of a wooden spoon (what i use most of the time, but you can use anything that will make about a 1cm hole) and poke rows of holes over the whole top of the cake.
Take the can of sweetened condensed milk and make two holes in the top, opposite of each other, and pour it into the holes on the cake until the can is empty.
Repeat step 4 with the caramel topping after heating it in the microwave for about 30 seconds (with the lid off)
Let the cake soak up the sweetened milk and caramel then add a the whipped topping as a frosting for the cake. You can add the crushed heath candies on top of the whipped cream here as well