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Separate the egg yolks from the whites. Beat the yolks with the sugar until pale and creamy. Add the lemon (optional) and the potato starch and beat.
In a separate bowl, whisk the egg whites until stiff peaks form, then delicately fold the whites through the batter.
Place the mixture into a round cake tin (buttered and lined with baking paper). Bake at a moderate heat for about an hour or until golden on top and firm to the touch.