Break the chocolate into pieces. Separate 2 eggs, set aside the egg whites. Put the two egg yolks, 1 whole egg and the chocolate in a saucepan, melt over very low heat and stir to a cream. Remove from heat, stir in the cognac and half of the vanilla sugar.
Beat the egg whites, add the remaining vanilla sugar and beat until stiff. In another bowl, whip the cream until stiff as well. Carefully fold the chocolate cream with the cream into the egg white. Pour into a bowl and set in the freezer for about 4 hours.
Take out of the freezer 1/2 hour before consumption and place in the refrigerator.