Prepare the cake mix following the instructions on the box and bake in a 9X13 pan.
While the cake is still warm, poke holes in the cake using the end of a wooden spoon.
Pour the can of sweetened condensed milk over the entire cake, making sure it soaks into the holes.
Warm the hot fudge sauce in the microwave for 30 seconds, then pour over the cake, making sure it soaks into the holes.
Once the cake cools, spread the container of Cool Whip on top.
Garnish the cake with chocolate chips.