For the grape strudel, whisk egg yolks with powdered sugar, ground cinnamon and lemon zest until foamy. Fold in the whipped snow with sugar and finally fold in flour and grated hazelnuts.
Spread 2/3 of the dough with this mixture. Sprinkle washed and dabbed grapes on top and drizzle the remaining third of the dough with butter.
Roll the whole into a strudel and bake the grape strudel.