Beat egg yolks, sugar and vanilla sugar until frothy. Add the quark and custard. Beat egg whites until stiff and fold in. If you use apricots, then first divide the quark mixture and spread it on the strudel dough, then spread the fruit over the mixture. But if you use berries, then fold them into the mass. Close and brush with milk. Bake at 180 ° C for about 40 minutes.