Put flour, 125 g sugar, vanillin sugar, salt, cocoa, egg and butter in flakes in a mixing bowl. Knead first with the dough hooks of the hand mixer, then with your hands to a smooth dough. Wrap in foil and chill for about 45 minutes.
Halve the walnut kernels lengthwise. Roll out the dough on a floured work surface to a thickness of about 3 mm. Cut out circles (4 cm Ø), knead the rest of the dough again and process in the same way. Place on 3 baking trays lined with baking paper.
Beat the egg white with the whisk of the hand mixer until stiff, add lemon juice, pour in 200 g sugar. Keep beating until it is shiny white. Pour the tuffs in portions into a piping bag with a perforated spout and spray them onto the cut-out dough circles (leave a rim of approx. 2 mm, as the egg white will rise during baking). Place a piece of walnut in the middle of each circle.
Bake one after the other in a preheated oven (electric oven: 175 °C/ convection oven: 150 °C/ gas: stage 2) for 12-15 minutes. Remove from the oven, carefully place on a cake rack, allow to cool and pack in tins