Preheat oven to 350 (or whatever temp it says on the cake box)
In a large bowl, combine all the ingredients for the cake plus vanilla extract and cornstarch. Bake according to package directions in a 9×13 baking pan.
Poke holes into the top of the cake using the end of a wooden spoon or a large meat fork.
In a small bowl, combine about 3/4 of the ice cream topping (save the other 1/4 for later) with sweetened condensed milk. Mix well.
Pour over the top of the cake, spreading if necessary to cover the whole cake. Then, spread Cool Whip on top.
Sprinkle with chopped Heath bars and drizzle with remaining ice cream topping. Chill until serving.
