Mix flour, baking powder and salt in a medium-size bowl; set aside. Using an electric mixer on high speed, beat 1/2 cup of the butter in a large bowl for 1 minute or until creamy. Add granulated sugar and continue to beat until light and fluffy.
Add eggs and vanilla, and beat until blended. Reduce speed to low. Gradually blend in flour mixture, then sour cream.
Melt remaining 5 tablespoons butter in an ovenproof 10-inch nonstick skillet. Add brown sugar; stir 2 minutes.
Add apple slices to the pan and reduce heat to medium-low. Cook for about 10 minutes or until tender. Let cool for 5 minutes.
Spoon batter over apples and spread to edge of pan. Bake at 375°F for 30 minutes or until golden brown. Cool on rack for 20 minutes. Run knife around edge. Carefully invert onto a plate.