Prepare the devil's food cake mix according to package directions, using a 9x13 pan. You'll probably need three eggs, oil and water for this.
While the cake is still hot: Use the straight end of a wooden mixing spoon, poke holes all over the cake, about half an inch apart. (See photos above.)
Immediately pour the sweetened condensed milk evenly over the top, letting it soak into the holes. Use the back of a spoon or spatula to gently nudge the milk to all of the edges and to every hole.
Drizzle the caramel topping slowly over the cake. If the caramel topping is particularly stiff, heat it for 20 seconds or so in the microwave first. Stir and pour over the cake. Again, coax caramel to every hole, if needed.
Cool completely to room temperature for about an hour.
On the cooled cake: Spread the thawed Cool Whip edge to edge on top of the caramel.
Sprinkle about 3/4 of the bag of Heath Bar bits over the top evenly, keeping some back to serve on the leftovers. (If you think there might be leftovers, if not, just add it all.)
if serving immediately, expect a gooey scoop of cake. For cleaner slices, cover and slide into the fridge for about 45 minutes to help it firm up.