Butter and lightly flour a 25 cm (10 inch) round springform tin.
Line the base with a round of greaseproof paper.
Sift the potato flour and the plain flour together twice.
Whisk the egg yolks with three-quarters of the castor sugar and lemon rind in a large mixing bowl until they are very thick.
Whisk the egg whites until stiff, but not too dry, then gradually whisk in the remaining castor sugar, whisking until very shiny.
Carefully fold the sifted flours into the egg yolk mixture, then gradually fold in the egg whites.
Pour the sponge mixture into the prepared tin and bake at 180ºC (350ºF) mark 4 for 40-45 minutes until very well risen, and a wooden cocktail stick or skewer inserted in the centre of the sponge comes out clean.
Allow the sponge to cool in the tin for 10 Minutes, then transfer to a wire rack to cool completely.
