Place over medium heat the sugar with the eggs and mix until it gets thicker.
After cooling add the soft butter, lemon zest, lemon juice and mix it until homogenized.
(optional) Place over medium heat the water, white sugar , vanilla sugar and mix it until the sugar is melted.
Spread the lemon filling on the layer, than place another cake layer on the top of the filling and continue until you are out of cream. Optionally, the sheets can be sprinkled with sugar syrup. As toping, scrape white chocolate.