Preheat oven 375F. Grease and flour a 9 in. Tart or pie pan. In a mixing bowl, combine flour, 2 T sugar and baking powder. With a pastry blender or 2 knives, cut in 1 1/2 T margarine. Add egg whites, milk and vanilla; beat until smooth. Batter will be stiff. Spoon into prepared pan and spread batter to fit pan. Arrange apple slices, overlapping, in 2 concentric circles to cover batter. In a small bowl, combine 3 T sugar and cinnamon; sprinkle over apple slices. Dot with 1/2 T margarine. Bake 30 min. Cool 10 min before serving.
