Sift the flour and powdered sugar into a bowl. Add butter, lemon zest, eggs and baking powder and work everything into a smooth dough. Wrap the dough in foil and let it rest in the refrigerator for about 30 minutes.
Roll out the dough about 2.5 mm thick on a floured worktop. Cut out circles with a cutter, approx. 6.5 cm in diameter. Put some Nutella in the middle with a teaspoon and place a second cookie on top. Seal the edges tightly so that the Nutella does not leak during baking. Brush the biscuits with egg yolk and sprinkle with chopped hazelnuts.
Line a baking sheet with parchment paper and place the biscuits on top. Bake in the oven preheated to 180 ° C for about 8 minutes.
If you don`t like Nutella, you can also use jam. You can also replace the hazelnuts with granulated sugar.