Although this requires some time to prepare, the finished product compensates for the effort made.
Separate eggs and allow whites to sit at room temperature for 1 hour. Melt chocolate and butter together in a saucepan. Remove from heat. Beat yolks, one at a time, into the hot mixture. Let cool 15 to 20 minutes. Stir in cognac. Whip egg whites until stiff. Fold cooled chocolate mixture into whites. Whip heavy cream and fold in also. Cover and refrigerate until firm. Decorate with whipped cream and shaved chocolate.