Preheat oven to 300 °F (150 °C).

Cream the butter with ⅓ of the sweetener.

Mix in the quark and then the egg yolks, little by little.

Beat the eggs to stiff peaks with the rest of the sweetener and fold into the quark mixture.

Finally, fold in ground almonds, lemon zest, and vanilla sugar.

Butter a Rehrücken pan and dust with whole wheat flour, fill with batter, and bake for about 30 minutes.

Let cool and remove from pan.