Preheat the oven to 180 degrees Celsius. Grease the baking tin with butter. Mix the flour, butter, 4 egg yolks, vanilla sugar and lemon zest into a smooth dough by hand or with a mixer with dough hooks. Roll ¾ of the dough into a sheet that is large enough for the baking tin and cover the tin with the sheet of dough. Cut away any excess dough. Fill the tart with the jam and cover it with lattices made from the remaining dough. Use the remaining egg yolk to coat the dough. Bake the tart in the oven for approximately 30 minutes until the dough is cooked and nice and brown, and the jam is firm and sticky. Allow the tart to cool before slicing.