For the curd strudel prepare a dough from flour, baking powder, sugar, vanilla sugar, salt, egg, milk and butter and let rest for 30 minutes.
For the filling, mix the butter with the sugar, vanilla sugar, salt, yolks and curd until smooth, fold in the sour cream and the firm beaten egg whites.
Roll out the dough to a thickness of 5 mm and spread with the filling, fold in the edges of the dough, brush the dough with egg yolk. Place on a baking tray lined with baking paper and bake at medium heat until golden brown.
Sprinkle with sugar.