Beat cold water and egg yolks together until mixture makes 1 litre, at least 10 minutes. Slowly add sugar and vanilla to egg yolk mixture, beating for at least 7 minutes.
Sieve the flour, cornflour, baking powder and salt together. Fold into the egg yolk mixture.
Beat egg whites until stiff peaks form and fold in smoothly to the egg yolk mixture. Pour batter into an ungreased 23 or 26cm tube cake tin.
Bake at 180 C / Gas 4 for 50 minutes. Turn upside down to cool in tin for 1 hour. Loosen sides and coax from tin. May be cut into layers and filled with lemon filling and iced with the same or with whipped cream.