Separate the egg whites and yolks. Mix yolks with half de sugar and beat until it has a creamy texture.

Beat the whites to hard peaks, adding the sugar slowly and beating after each addition. Pre-heat the oven to 170-180 celsius (338-356 F)

Mix the yolks with the whites in slow movements. Add the flour, sieved. Once it's integrated completely add the butter. Add the vanilla extract.

Cook for 30-40 minutes. It does not have to rise much.