For the grape strudel, cream the room-warm butter with hazelnuts, powdered sugar, vanilla sugar, a pinch of salt and lemon zest. Gradually beat in the egg yolks.
Beat egg whites with granulated sugar until semi-stiff and fold into the butter mixture.
Place the prepared strudel dough on a floured kitchen towel. Brush with butter, spread filling and scatter grapes over the top.
Cut off irregular edges of dough and carefully roll into a strudel. Close ends tightly and place seam side down on a baking sheet lined with parchment paper.
Brush generously with butter and bake at 170 °C for 30-40 minutes.
Remove, let cool and sprinkle with powdered sugar before serving.