Mix the butter, sugar and eggs together until the mixture is frothy. Add the vanilla sugar, salt and lemon juice, keep stirring. Gradually add the quark and stir until everything is even (be careful, be sure to put on a kitchen apron beforehand, because it splatters terribly). Stir in the flour and baking powder. Put the dough in a greased springform pan and bake at 180 ° C for about 1 hour until the Ursel is nice and brown.