Line a baking tray with baking paper. Beat the egg whites and 1 pinch of salt until stiff, adding icing sugar and cocoa. Fold in the nuts.
Pour the nut mixture onto the baking tray and spread it into a square (approx. 27 x 27 cm). Let it dry overnight in a cool, dry place.
Cut the nut mixture into 81 small squares (approx. 3 x 3 cm), remove them from the baking paper with a pallet or spatula and spread them on two baking trays lined with baking paper. Bake one after the other in the preheated oven (electric oven: 150 °C/circulating air: 125 °C/gas: level 1) for approx. 25 minutes.
Let it cool down.