Grease Springform pan. Wrap with foil on outside. Spread brown sugar evenly on bottom of pan. Press evenly. Sprinkle 1 tsp cinnamon on brown sugar. Cut 2 tbsp butter into bits and scatter over sugar.

Arrange apples (halves) in pan overlapping. Sift flour, baking powder, soda, salt and 1 tsp cinnamon. Set aside. Cream sugar and 1/2 cup butter. Add egg, beat until fluffy. Add buttermilk and vanilla. Mix well (might appear curdled). Add sifted ingredients, mix until smooth. Spread over apples carefully. Place pan on baking sheet. Bake at 350F for 1 hour. Let cake cool for 5 minutes. Remove ring and invert onto serving plate. Leave bottom of pan on for 10 more minutes. Lift bottom off and let cook for 1 hour before serving. Can be made 1 day ahead. Keep at room temp. Reheat at 200F. Serve with cool whip and sprinkle with cinnomon