In a small bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Beat in egg. Beat in vanilla. Combine the flour, baking powder and salt; add to the creamed mixture alternately with sour cream, beating well after each addition. Fold in chopped apple; set aside., In a 7-in. cast-iron or other ovenproof skillet, melt butter and chips with brown sugar over medium heat. Stir in sliced apple. Spoon and spread batter over apple mixture. , Bake at 375° until a toothpick inserted in the center comes out clean, 20-25 minutes. Cool for 5 minutes before inverting onto a serving plate. Serve warm.