Preheat oven to 375˚F.
Combine the cinnamon and brwn sugar in a small dish. Set aside.
In a large bowl, whisk together the eggs and milk. In a separate bowl, mix the flour and baking powder together. Add the flour mixture to the egg mixture and stir.
Melt the butter in an open-proof skillet over medium heat. Place the apple slices in the skillet and sprinkle with 1 tablespoon of the brown sugar-cinnamon mixture. Cook, stirring occasionally, about 5 minutes. Pour the egg mixture over the apples, then sprinkle evenly with the remaining brown sugar mixture. Bake 18-20 minutes, until lightly golden and puffy. Serve hot, sprinkled with some powdered sugar if you'd like.