For the batter, beat eggs, sugar, vanilla sugar and lemon zest until foamy. Alternately stir vegetable oil and 125 ml water into the egg mixture. Mix flour with baking powder and fold in.
Line a baking tray with baking paper and spread the batter on it.
For the curd topping, separate the eggs. Mix the curd, yolks, sugar and vanilla sugar well. Beat the egg whites until stiff. Carefully fold into the yolk mixture.
Pour the curd mixture into a piping bag with a large nozzle and pipe onto the dough in a grid pattern.
Bake in the preheated oven at 180 °C for approx. 30 minutes.
Serve the quilt cake sprinkled with powdered sugar.