Mix the egg yolks with the soft butter, sugar and flour and spread on a baking sheet lined with baking paper.
Beat the egg white until stiff and carefully fold in the powdered sugar, hazelnuts and grated chocolate. Spread the mixture on the dough and bake at 200 ° C for about 15 minutes.
Carefully cover the cold cake with melted dark chocolate. When the chocolate is firm, cut the cake into small pieces and serve.