Heat oven to 375. Very lightly oil a oven proof cast iron pan. Peel apples and core and slice them. Mix apples and cinnamon and sugars in a bowl. Mix flour, baking powder and salt in a medium bowl. Stir in buttermilk and eggs, mix well. Melt butter in a pan add apple mixture and cook stirring until sugars are melted and apples are well coated about 5 minutes. Pour egg mixture over apples. Bake until puffed and edges brown and apples are tender about 30 minutes. Serve immediately.