Beat egg yolks, sugar, lemon juice or water, lemon rind and salt until light and fluffy (5 minutes at high mixer speed). Sift flour and cornflour, and mix into egg yolk mixture a little at a time. Whisk egg whites until soft peaks form, and fold gently into yolk mixture.
Divide mixture into 2 unbuttered cake tins. Cut through mixture gently several times to break up any large air bubbles.
Invert layers on wire racks. When cool, loosen edges and remove from tins.