Preheat your oven to 350°. Spray a 9x13 baking dish with nonstick cooking spray.
Use a large bowl to mix the cake batter by following the instructions on the box.
Add the batter into the pre-greased baking dish and bake according to the package instructions.
Once the cake is done baking, remove it from the oven and use the end of a wooden spoon (or a chopstick, or anything small and round), and begin poking holes all over the warm cake.
Pour the can of sweetened condensed milk all over the cake, and use a spatula to gently press it down into the holes you poked.
Next, pour the caramel sauce all over and gently smooth it into the holes in the cake. Sprinkle one cup of the broken heath bar pieces over the top of the cake.
Cover the dish loosely with plastic wrap and place it in the refrigerator for at least 2 hours.
Remove the chilled cake from the refrigerator and add the cool whip topping all over the top of the cake. Spread it out so that the entire cake is covered.
Sprinkle on the remaining crumbled Heath bar pieces and serve immediately!