Separate the eggs. Mix the egg yolks, quark, starch, powdered sugar, lemon zest and vanilla sugar together. Beat the egg whites until stiff and pour in the sugar. Fold the egg whites under the curd mixture. Mix in 2/3 of the berries as well. Grease a flat baking dish with the butter and fill it with the quark mixture. Scatter the remaining berries on top. Bake at 175 degrees for about 40-50 minutes.
After baking, sprinkle with powdered sugar and serve hot or cold.