Prepare cake according to directions on box. While the cake is still warm, use the end of a wooden spoon to poke holes throughout the cake. Pour the sweetened, condensed milk and caramel ice cream topping over the cake while it is still warm. Allow to cool before frosting with the cool whip. Sprinkle crushed Heath candy bars over the top. If not serving immediately, keep refrigerated. You can also wait to frost and decorate until serving time, which will allow the cake to be served at room temperature.