Separate egg. Beat egg yolks in a bowl with coffee, liqueur and sugar over a hot water bath until creamy. Remove from heat, stir in chopped chocolate until melted, then beat until cold. Whip approximately half of cream until stiff. Beat egg whites until stiff, fold into coffee mixture and pour into 4 freeze-resistant dishes. Cover and freeze for at least 4 hours.
Before serving, remove from freezer for 10 minutes. Whip remaining cream with vanilla sugar until stiff, cover frozen coffee mixture with whipped cream and garnish with chocolate.