Preheat the oven to 170 ° O / bottom heat. Beat eggs, sugar, vanilla sugar and lemon zest until whitish-creamy. As you continue to whip, slowly pour in the oil. Then fold in the yogurt. Mix both flours with the baking powder, sieve into the egg mixture and stir in only very briefly on the lowest level until no more flour is visible. Fold in the blueberries and fill each well to the brim. Sprinkle the hazelnut brittle on the surfaces and slide the tins into the oven on the grid on the 2nd rail from below. The baking time is 35 minutes.