Preheat your oven to 350 degrees F and grease a 9x13 baking dish.
Start by making the cake. Mix the cake mix, water, vegetable oil, and eggs in a large bowl.
1 box (15.25 ounces) chocolate cake mix, 1 ½ cups water, ½ cup canola oil, 3 eggs
Once the cake batter is nice and smooth, pour it into the prepared cake pan and bake the cake for 30 minutes. Stick a toothpick in the center of the cake to check if the cake is fully baked. The toothpick will come out of the cake cleanly when baked.
Let the cake cool in the pan for about 15 minutes then flip it out onto a large plate.
Cut shallow slits across the top of the cake in a grid pattern. The slits should go about ½ way down into the cake.
Mix the sweetened condensed milk and caramel sauce together in a small saucepan. Heat over low heat for 2-3 minutes. Pour the warm mix over the top of the cake slowly, letting it absorb into the cake. It's okay if some drips down the sides, too.
1 can sweetened condensed milk, 1 cup caramel sauce
Chop the Heath bars into small pieces and sprinkle half of the pieces over the top of the cake.
3 large Heath Bars (chocolate toffee bars)
Spread the Cool Whip over the top of the cake and then sprinkle the remaining toffee pieces on top.
8 ounces Cool Whip, thawed
Refrigerate the cake until you are ready to serve then slice and enjoy!