Preheat oven to 350F. Line a 9×13-inch pan with aluminum foil for easier cleanup and spray with cooking spray; set aside.
Make cake according to package directions (likely adding 3 eggs, water, and oil) and bake as directed. If you’re adding the optional coconut extract, add it after adding the eggs, water, and oil.
While the cake bakes combine the sweetened condensed milk, caramel sundae topping, and whisk to combine; set aside.
Remove cake from the oven when it’s done (probably about 23 to 28 minutes) and use a fork to poke holes all over the cake. I didn’t count but I estimate I poked the cake in about 100 places, evenly spaced over the surface.
Slowly pour the caramel mixture over the cake, taking your time to evenly distribute it. Place cake in the fridge for about 15 minutes to cool slightly before adding the whipped topping so it doesn’t melt.
Remove cake from the fridge, evenly spread the whipped topping over the surface, and cover cake and refrigerate for at least 2 hours (overnight is best).
Before serving, add the coconut to a small dry nonstick skillet and toast it over medium heat on the stove. Stir it nearly constantly and keep a very close eye on it because it’s very prone to burning. It will likely be fully toasted in about 2 to 3 minutes.
Evenly sprinkle the coconut over the cake and serve.