Beat the eggs until frothy and gradually add the powdered sugar, vanilla sugar and a pinch of salt. Hit through well. Keep beating and then slowly adding the egg liqueur and then slowly adding the oil. Beat for another 3 minutes until a very foamy mass has formed.
Mix the flour with the baking powder and sift it into the mixture with a whisk. Pour the dough into the mold and bake on the middle rack for about 60-70 minutes (make a stick test). Take out and let cool for about 10 minutes, turn over.
If you like, you can sprinkle powdered sugar over it when the cake has cooled down.
Tip: it becomes even looser if you use 100g flour and 150g cornstarch instead of 250 g flour.