Separate the egg whites from the yolks and beat the whites until stiff peaks form.

Beat the soft butter with the sugar, vanilla sugar, and lemon zest until fluffy, then add the yolks.

Mix the almond flour, starch, baking powder, and cinnamon.

Alternately add the milk and flour mixture to the butter-yolk mixture.

Finally, gently fold in the whipped egg whites, and if necessary, add the sparkling water.

This makes a lot, so for a small group of people, divide the recipe in half.