Preheat the oven to 350°F. Butter and flour a 9-inch round cake pan, or line the bottom with a round of parchment paper.

Combine the eggs and sugar in a bowl. Unless the egg mixture already feels warm, put the bowl over a saucepan of barely simmering water and whisk until warm and frothy.

Beat on high speed with a stand mixer fitted with the whisk attachment for 4 minutes, with a handheld mixer for 20 minutes, or by hand using a bowl and balloon whisk for 12 minutes, or until the ribbon stage: when the beater is lifted, the mixture falls in a wide band onto the surface, forming a figure eight that stays for 5 seconds before dissolving.

Carefully transfer the egg mixture to a large bowl. Fold in the flour by sifting it over the beaten eggs and folding with a rubber spatula.

In a smaller bowl, fold together the melted butter and about one-fifth of the egg mixture. Fold the butter-egg mixture into the egg mixture in the larger bowl.

Transfer the batter to the prepared pan and gently smooth the top with an offset spatula. Bake for about 25 minutes, or until the cake bounces back to the touch and a toothpick inserted into the center comes out clean.Let cool for 5 minutes in the cake pan and then turn out onto a cake rack.

For Chocolate Sponge: Add 4-1/2 tablespoons cocoa powdered and 2 extra tablespoons sugar