1.Position oven shelves; preheat oven to 180°C. Grease two deep 20cm-round cake pans.
2.In a large bowl, beat eggs with electric mixer until thick and foamy. Gradually add sugar, about a tablespoonful at a time, beating until sugar is dissolved between additions. Total beating time should be about 10 minutes.
3.Sift flour and cornflour together three times onto paper.
4.Sift flour mixture over egg mixture. Then, using one hand like a rake, quickly and lightly fold and pull flour mixture through egg mixture, using the side of your hand as a scraper to make sure all the ingredients are combined.
5.Pour combined butter and the water down side of bowl; using one hand, fold through egg mixture. Pour mixture evenly into prepared pans, using metal spatula, spread mixture to edges of pans.
6.Bake sponges about 25 minutes. As soon as sponges are baked, turn onto wire racks covered with baking paper; turn top-side up to cool.
7.Place one sponge on serving plate, spread with lemon butter and whipped cream. Top with remaining cake, dust with sifted icing sugar.
Beating the eggs and sugar well helps correctly aerate the mixture. Caster sugar dissolves the fastest; while crystal sugar can be used, it requires longer beating to be incorporated. It is important to create volume in the egg and sugar mixture. Use a small bowl with deep sides and beat the mixture with either a hand-held or stand-mixer. You will get better results this way, even though you have to transfer the sponge mixture to a larger bowl to fold in the flour and water. If you don’t like using your hand to mix ingredients, use a rubber or plastic spatula, or a large metal spoon. When the sponge is cooked, it will feel springy when touched gently with fingertips, and will have shrunk slightly away from the side of pan. Turn the sponge from cake pan as soon as it is baked, or heat from the pan will continue to cook it, giving it a too-crisp crust. Sponge is best made on day of serving. Refrigerate if it is to be filled more than an hour ahead. Unfilled sponge can be frozen for up to 1 month.