Beat together the egg yolks and the sugar to form a white, frothy mixture.
Add the flour, potato starch and lemon zest. Beat the egg whites until they are very stiff, and then add them to the mixture one spoon at a time, blending them very gently to avoid ‘breaking' them.
Grease a cake tin and pour in the mixture.
Bake in the oven at gas mark 6 (180°) for 45 minutes.
Check that the cake is cooked by inserting a skewer - it should come out clean. Carefully remove the cake from the mould. It can be sprinkled with icing sugar or served as it is.

