For the dough, mix the butter, powdered sugar, vanilla sugar and salt together well until it is creamy. Stir in the egg, then work in the flour. Shape the dough into a ball, exerting little pressure, wrap it in foil and let it rest in the refrigerator for about 2 hours.
Preheat the oven to 180 ° C. Roll out the dough and put in a buttered form. Prick the dough base several times with a fork. Bake blind for about 10 minutes, then bake until golden brown in 10-15 minutes, then let it cool down (note: it can also be baked without blind, I baked the dough in one step for 25 minutes).
For the filling, whisk together the lemon zest and juice, sugar and eggs. Warm up slowly in a water bath, stirring constantly! It must not boil, otherwise the mixture will turn to scrambled eggs! After about 20 minutes the mass thickens nice and creamy. Remove from heat, add pieces of cold butter and beat with a hand blender for about 2 minutes until airy and creamy. Place on the base and cover in the refrigerator for at least 1 hour.