In a large bowl, mix together the butter and sugar until smooth. Add the egg yolks and beat until light and fluffy. Sift in the cake flour, baking powder and salt, and stir until well blended. Mix in the vanilla and milk. In a separate bowl, whip the egg whites to stiff peaks using an electric mixer. Fold into the batter. Pour into the prepared pie plate, and sprinkle cinnamon sugar over the top.