One caveat from the Cake Doctor is that you cannot shortcut and use canned frostings on your cake. I would rather serve a cake plain. Use regular salted butter to balance the high sugar content of cake mixes and give a more homemade taste. If you must use margarine for health reasons, make sure it is stick style and has more than 65 percent vegetable oil. This is similar in flavor to a key lime pie, but Byrn makes a quick and easy crust from cake mix.
Some cake mix goes into the filling, too, making it easy to slice and serve.
Place the remaining cake mix, butter and 1 egg in a large mixing bowl. Blend with electric mixer on low speed for 2 minutes. Stop the machine and scrape down the sides of the bowl with a rubber spatula. The batter should come together in a ball. With your fingertips, pat the batter evenly over the bottom and 1 inch up the sides of the prepared pan, smoothing it out with your fingers until the top is smooth.