Mix butter, sugar, egg yolks, vanilla sugar, flour, hazelnuts, cognac and cocoa powder for approx. 10 minutes to form a smooth mixture. Squeeze the lemon and add the juice to the egg whites and beat until stiff, then fold into the batter.
Put the mixture in a large, buttered springform pan and bake at 125 ° C - 150 ° C for 1 - 1 1/4 hours.
The cake can be decorated with powdered sugar or chocolate icing, but it doesn`t have to, it tastes so delicious too.