Chop chocolate coarsely and melt together with cognac and espresso over a hot water bath. Remove from heat.
Separate eggs, beat egg yolks with vanilla sugar until fluffy and sugar has dissolved. Beat very cold heavy cream until stiff. Beat egg whites with sugar until stiff.
Mix egg yolk mixture with melted chocolate and stir with a whisk. Add whipped cream to chocolate mixture and whisk quickly, mixing before it solidifies.
Fold in egg whites gently. Pour into glasses, cover tightly with foil and refrigerate for at least 3 hours. Before serving, remove foil and sprinkle with grated chocolate if desired. Serve with fresh coffee.