Clarify 1 lb. fresh butter; when cold beat it to a cream, add 12 oz. white sugar, 1 lb. potato flour passed through a sieve, four whole eggs and the yolks of two more, the best of one lemon and a few drops of lemon or vanilla essence. Beat the whole for at least one hour, and until it makes bubbles. Bake in flat round or square, shallow mould, buttered and finely breadcrumbed, in a moderate oven. Dust over with fine white sugar - icing sugar, for choice - before serving.