Butter custard can be made either with egg yolks or egg whites.
With egg yolks: Cook the sugar, barely covered with water, over a moderate heat, until a drop falling from the fork forms a thread. Whip the egg yolks while adding the boiling syrup very slowly until they whiten and fall from the fork in ribbons. Beat further until totally cooled down. Work the butter into a paste, add the egg and sugar mixture, as well as the vanilla sugar, little by little. The custard is ready to use, all you have to do is to add the chosen flavor: coffee, chocolate, almond, etc.
With egg whites: Make the custard exactly in the same way, but pour the boiling syrup over the firm stiffly beaten egg whites. Butter custard is the classic filling of a large number of cakes: Genoese sponge, meringue, yule log.
