Chop chocolate coarsely and melt together with cognac and espresso over hot water bath. Remove from heat.
Separate eggs. Whisk egg yolks with vanilla sugar over hot water bath until frothy and until sugar has dissolved.
Beat well-chilled cream until semi-stiff.
Beat egg whites with sugar until stiff.
Add egg yolk mixture to chocolate mixture and whisk well. Fold in cream.
Fold in egg whites gently and place mixture into a bowl.
Tightly cover with foil and refrigerate for at least 3 hours. Serve.