Preheat oven to 350°F. Lightly coat a 9x13-inch baking pan with non-stick cooking spray.
Pour the prepared cake batter into the pan and bake according to package directions for a 9x13-inch cake.
When a toothpick inserted into the center comes out clean, remove the cake from the oven.
Using the straight end of a wooden mixing spoon, poke holes all over the cake, about half an inch apart.
While the cake is still hot, pour the sweetened condensed milk evenly over the top, letting it soak into the holes.
Drizzle the caramel topping over the cake.
Cool completely to room temperature for about an hour.
Spread the Cool Whip evenly over the cooled cake.
Sprinkle the Heath Bar bits over the top (use half or the full bag, as desired).
Slice and serve immediately, or chill before serving.