Heat the oven to 350°F and grease a 9-by-13-inch baking pan. Prepare the cake mix with the eggs, vegetable oil, water, and any other ingredients required by the package directions.

Spread the batter evenly in the prepared pan and bake for 28 to 35 minutes, or until a toothpick inserted near the center comes out clean.

Let the cake cool for 5 to 10 minutes. Use the handle of a wooden spoon or a large straw to make evenly spaced holes about 1 inch apart across the cake.

Slowly pour the sweetened condensed milk over the warm cake, aiming it into the holes. Drizzle the caramel topping over the surface and spread it gently so it covers the cake.

Cool the cake completely, then spread the thawed whipped topping over the top. Sprinkle the chopped candy bars evenly over the whipped topping.

Refrigerate the finished cake for at least 1 hour before serving. For the best texture, chill it overnight and add a little extra chopped candy just before serving if desired.