1. Chop the chocolate into pieces and melt over a bain-marie. Add the butter, stir until smooth. Set aside to cool.

2. Whip the cream to stiff peaks. Beat the egg yolks and blend into the cream. Whisk the egg whites until frothy, then, while still whisking, add a third of the sugar a little at a time until the egg whites are smooth and shiny. Gradually add the remaining sugar, whisking until stiff peaks form.

3. Carefully fold the egg whites into the cream mixture, one third at a time. Fold in the melted chocolate mixture, making sure it is thoroughly incorporated. Spoon the mix into 4 cups or glasses and refrigerate for at least 3 hours.

4. Whip some additional cream and place a dollop on top of the mousse. You can also grate chocolate over the cream, a classic way to finish a classic dessert.