Reserve 2 tablespoons of the sugar, and combine the rest of the sugar with the egg yolks in a stainless steel bowl. Place the bowl in a skillet of hot tap water (or use a double boiler), and whisk the mixture for 3 minutes, or until it is fluffy, smooth, and at least doubled in volume.

Beat the reserved sugar with the cream in a large chilled bowl for a few minutes, or until soft peaks form; do not overwhip. Transfer about 3/4 cup of the whipped cream to another bowl to use as a decoration, and refrigerate.

Using a rubber spatula, combine the melted chocolate with the yolk mixture and the cognac. If the mixture starts to seize or break down, immediately stir in 1 to 2 tablespoons of the whipped cream to smooth out the mixture. Gently fold in the remaining whipped cream until incorporated. Transfer the mousse to a decorative bowl, cover, and refrigerate until set, at least 2 hours.

At serving time, whip the reserved 3/4 cup whipped cream until stiff peaks form. Spoon the cream into a pastry bag fitted with a star tip and decorate the top of the mousse with the cream, or spoon dollops of the cream onto the top of the mousse. Serve.