Preheat your oven to the temperature specified on the cake mix box, typically around 350°F (175°C). Grease a 9x13-inch baking pan thoroughly with cooking spray or butter.

In a large mixing bowl, combine the vanilla cake mix with the required eggs, oil, and water as per the package instructions. Gently fold in the pumpkin puree until smooth.

Pour the batter into the prepared baking pan and bake for 30 to 35 minutes, until a toothpick inserted into the center comes out clean.

Remove the cake from the oven and let it cool completely in the pan on a wire rack for about 1 hour.

Once the cake is fully cooled, poke holes all over the surface at regular intervals, about 1 inch apart.

Pour the sweetened condensed milk evenly over the cake, allowing it to seep into the holes, and let it sit for a few minutes.

Spread the Cool Whip evenly over the cooled cake.

Drizzle the caramel sundae topping generously over the Cool Whip.

Sprinkle the Heath candy bits evenly on top of the caramel.

Refrigerate the assembled cake for at least 2 hours before serving.