Preheat the oven to 350°F and line a 7-inch round cake pan with parchment paper.
In a mixing bowl, cream the margarine or butter with the sugars until light and fluffy. Separate the eggs and beat in the yolks only. Sift in the cake meal, potato flour, and baking powder, and fold in carefully. In a separate bowl, whisk the egg whites with a pinch of salt until soft peaks form. Add a spoonful of the egg white to the cake batter and mix in thoroughly. Now fold in the rest of the eggs whites, taking care not to knock out any air. Spoon the mixture into your prepared pan and bake for 35-40 minutes until risen and golden.
After the cake has cooled, slice in half horizontally and spread the bottom half with jam. Replace the top half and sprinkle superfine sugar over the top. Enjoy!
