Bake the cake according to the package instructions and let it cool slightly.
While still warm, poke holes all over the cake using the end of a wooden spoon or a large fork.
Pour the sweetened condensed milk evenly over the cake, letting it soak into the holes.
Follow with a layer of caramel sauce, spreading it gently with a spoon or spatula.
Once the cake is completely cool, spread the whipped topping evenly over the surface.
Sprinkle crushed Heath bars generously across the top.
Chill the cake in the fridge for at least an hour before serving (if you can wait that long).