100 g / 3 1/2 oz (7 tablespoons) unsalted butter, melted and cooled, plus extra for greasing
6 eggs, separated
200 g / 7 oz (1 cup) caster/superfine sugar
zest of 1 lemon, grated
800 g / 3 oz (3/4 cup) plain/all purpose flour
50 g / 2 oz (1/2 cup) potato flour
Grease a 25-cm/10-inch springform cake tin with butter and line the base with baking parchment.
Whisk the egg yolks, all but 1 tablespoon sugar and lemon rind until light then fold in the butter.
Sift the plain and potato flours and a pinch of salt on to greaseproof paper.
Whisk the whites in a grease-free bowl until just stiff. Whisk in the remaining sugar. Fold the flour and whisked whites alternately into the egg/sugar mixture.
Spread into the tin.
Bake for 30 minutes or until a cocktail stick inserted in the centre comes out clean.