For the dough, beat sugar, egg yolks, vanilla sugar, salt and lemon zest until fluffy. Gradually stir in the oil and water. Finally, fold in the flour and beaten egg whites.
Spread the dough on a baking tray lined with baking paper.
For the curd mixture, mix all ingredients and fold in beaten egg whites.
Fill the curd mixture into a piping bag and pipe onto the dough in a lattice shape.
Bake at 170 °C for approx. 25-30 minutes.
Serve the cake cooled and sprinkled with powdered sugar.