Preheat the oven to 190 C / Gas 5. Grease and flour a 23cm (9 in) tart or flan dish.
Combine flour, 2 tablespoons of the sugar and baking powder. Cut 1 1/2 tablespoons of the margarine into bits and add to the flour mixture along with the egg whites, milk and vanilla. Beat till smooth and pour into prepared tin.
Arrange apple slices on top of the cake mixture, covering in a nice pattern. Combine the remaining 3 tablespoons sugar with cinnamon, then sprinkle over apples. Dot with remaining margarine.