Preheat your oven to 350°F. Butter down a 9x13 inch baking dish or coat it in nonstick cooking spray.
In a large mixing bowl, whisk together the cake mix, oil, water, and eggs.
Once the cake batter is homogenous, pour it into the prepared dish. Bake for 30 minutes.
Use a large skewer to poke holes throughout the cake. Stir together the condensed milk and caramel sauce and then microwave the mixture for 20 seconds.
Pour the caramel mixture over the top of the cake and then allow it to cool on the counter for 30 minutes.
Once the cake is cool, spread the cool whip over the top of it and sprinkle on the Heath bar pieces.
Slice, serve, and enjoy!
Notes
The more holes you poke in your cake, the more evenly the caramel mixture will seep into the cake.
Allowing your cake to cool helps prevent the cool whip from sliding off and gives caramel mixture time to seep into the cake.
You can store your leftover cake in the fridge for up to 4 days. Cover the pan with plastic wrap or transfer your cake slices to a large Rubbermaid container.
You can substitute the vegetable oil for either coconut or canola oil. You can also use 1/2 cup of melted butter (salted or unsalted).
You can find crushed heath bars at most major grocery stores on the baking aisle next to the chocolate chips.
To ensure the cool whip is thawed, I like to transfer it to the fridge when I start making the cake batter.