Preheat oven to 350°F. Line a 9×13-inch pan with aluminum foil (for easier cleanup) and spray the foil with cooking spray; set pan aside.

Make the cake batter according to the package directions for the 15.25-ounce cake mix (add the eggs, water, and oil the box calls for). If using the optional coconut extract (1 to 2 teaspoons), add it to the batter after you combine the eggs, water, and oil.

Pour the batter into the prepared 9×13-inch pan and bake according to the cake mix package directions.

While the cake bakes, combine the 14-ounce can sweetened condensed milk and 6 ounces caramel sundae topping in a bowl and whisk until smooth; set aside.

When the cake is done, remove it from the oven and use a fork to poke holes evenly across the top of the cake (about 1/2-inch apart so the mixture can soak in).

Slowly pour the sweetened condensed milk–caramel mixture over the warm cake, pouring gradually and allowing the liquid to sink into the holes and distribute evenly.

Place the cake in the refrigerator for about 15 minutes to cool slightly before adding the whipped topping so the topping does not melt.

Remove the cake from the refrigerator and spread the thawed 8-ounce container of whipped topping evenly over the surface. Cover the cake and refrigerate for at least 2 hours (overnight is best) to set.

Just before serving, toast 1 to 1 1/2 cups sweetened shredded coconut in a small dry nonstick skillet over medium heat, stirring nearly constantly and watching closely, about 2–3 minutes, until golden. Transfer the toasted coconut to a bowl to cool.

Evenly sprinkle the cooled toasted coconut over the chilled cake, remove the foil if used, slice, and serve.