Melt butter in a non-stick frying pan and add the apple slices and cinnamon. Cook over medium heat for 2–3 minutes until they begin to soften. Pour the prepared cake batter directly over the apples in the pan, spreading it evenly to cover all the fruit. Place a lid on the pan and cook on low heat for about 10 minutes. The steam trapped by the lid will help the cake rise and cook through without burning the bottom.