Preheat your oven to 350 degrees F. Flour and grease a 9-inch x 13-inch baking dish.
Prepare the cake mix according to package directions using egg, butter, and water. Pour the batter into the greased baking dish. Bake cake for 30 to 35 minutes. Carefully remove cake from oven and allow to cool for 20 minutes.
Reserve 1/4 cup of the caramel sauce. Then, in a medium bowl, stir together the sweetened condensed milk and remaining caramel sauce until combined.
Use the stick end of a wooden spoon to poke holes all over the cake about 1/2-inch apart. Pour the sweetened condensed milk mixture over the cake being sure to get it in all the holes. Use a rubber spatula to spread it evenly over the cake. Refrigerate for 2 hours.
While the cake is chilling, place the coconut in a large skillet over medium heat. Toast the coconut until golden, about 5 to 8 minutes. Stir frequently to prevent burning. Transfer to a bowl to cool.
Place the chocolate frosting in a microwave-safe bowl. Heat for 20 to 30 seconds, or until warm enough to pour. Remove cake from the fridge and pour the frosting all over the top of the cake. Use an offset spatula or the back of a spoon to spread the frosting evenly over the cake.
Sprinkle the top of your cake with toasted coconut until it's completely covered. Drizzle the reserved caramel sauce over the coconut. Refrigerate the cake for at least 2 hours.
Cut into 12 slices and serve. Cover any leftovers and store in the fridge for up to 3 days.
If you can't find butter cake mix, you can use vanilla or yellow cake mix instead. Cake ingredients and amounts will vary depending on the brand of cake mix and type of cake you purchase. Prepare your cake according to the package directions!