Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the cake mix, room temperature sour cream, and package directions to prepare the batter. Pour into a greased 9 x 13-inch pan.

Bake for 25-30 minutes or until the top is golden brown and a toothpick inserted in the center comes out clean. Cool completely on a wire rack for 1-2 hours.

Mix the reserved coconut flakes, sugar, and sour cream in a medium bowl until smooth. Refrigerate for at least 30 minutes, reserving 1 cup for later use.

Spread the remaining coconut mixture evenly over the cooled cake.

In a separate bowl, combine the reserved coconut mixture with whipped topping and mix gently. Spread this over the cake.

Cover with plastic wrap and refrigerate for at least 4 hours or overnight for the best flavor.