Beat the sugar and egg yolks for 15 minutes.
Sift the cocoa powder with the flour. Melt the clarified butter with the dark chocolate.
Beat the second part of the sugar with the egg whites.
Incorporate the melted butter and dark chocolate into the egg yolk and sugar mass.
Incorporate the sifted flour and cocoa.
Fold into the beaten egg whites.
Bake 1 hour at 160°C in the oven.