Mix powdered sugar and cocoa. Add the egg white and whisk smooth with the whisk of the hand mixer. Stir in hazelnuts. Line a baking tray (32 x 39 cm) with baking paper. Pour the mixture on top and smooth it down to a square (27 x 27 cm). Let dry overnight in a cool and dry place
Cut the mixture into 81 small squares (3 x 3 cm). Line 2 baking trays with baking paper. Remove squares from the baking paper with a pallet and spread them on the baking trays. Bake one after the other in a preheated oven (electric cooker: 150 °C/ convection oven: 125 °C/ gas: level 1) for about 25 minutes. Take out and let cool on the baking trays
Waiting time approx. 12 hours