In a cauldron kept in a bain-marie, whisk the whole eggs, egg yolks and sugar until warm. It is important that the water in the water bath does not boil.

Remove the bowl from the water and, when the mixture is cold, whisk it again, alternately, in a circular motion and from top to bottom, so that the mass swells.

Mix the flour and cornstarch together, then slowly drop them onto the puffy mixture, taking care to stir with a spatula or wooden spoon so that the ingredients are well blended.

Now add the melted butter, making sure it is not hot, very slowly, and the lemon zest, stirring to incorporate them into the mixture.

Pour the preparation into one or more buttered cake pans and bake in a hot oven.