For the sponge cake, beat eggs with sugar, vanilla sugar, salt and lemon zest until foamy. Sift flour and cornstarch and stir in loosely. Finally, fold in clarified butter or oil. Spread on a baking tray lined with baking parchment and bake quickly in the preheated oven at 220°C for about 5-6 minutes (bake the cake for about 10 minutes at 200°C and about 40 minutes at 160°C).
After removing, immediately sprinkle with a little flour or sugar. Place a second baking parchment and turn out immediately while hot. Let sponge cool between the baking paper so that it does not dry out. Fill and roll according to further use.