Preheat oven to 350 degrees. Grease a 9 by 9-inch square cake pan (can use a disposable foil cake pan) with 1 tbs butter.
Prepare cake mix as described on cake box, with 1/2 cup water and the egg.
Peel and dice apple into small bits. Sprinkle apples with cinnamon and sugar and mix well.
Pour cake batter into prepared cake pan. Spread apples on top of cake batter, evenly covering the entire top of the cake. ( I tend to put a little more towards the edges so that when the cake rises in the oven the apples don't fall to the center of the cake.).
Bake 20 to 25 minutes.
Best if served warm with ice cream. Very good at room temperature though.