Sift the flour, baking powder and salt into a bowl and set aside.
Peel and core the apples. Cut into thin slices.
In a stand mixer with a paddle attachment, cream the butter. Add the sugar and beat until light and fluffy. Beat in the egg.
Fold in the sifted flour alternately with the milk and mix well until the batter is smooth and even.
Spread the mixture into a well-greased 8-inch cake tin. Arrange the apple slices in a swirl on top and sprinkle with the combined cinnamon and extra sugar.
Bake for approximately 45 minutes.
Notes
If you have self-raising flour, use that instead of the flour and baking powder. In any case, make sure that your self-raising flour or baking powder have not passed their best before date.This cake is best eaten when it is still warm. If you like, serve with whipped cream. Please don't use whipped can fro a can - it will often be too sweet for the cake.