1. Break the chocolate into small pieces in a bowl. Melt in a bain-marie. Set aside off the heat, add the butter in small cubes.
2. Separate the egg whites from the yolks.
3. Beat the egg whites until stiff. Add the egg yolks to the melted chocolate, mix to obtain a smooth preparation.
4. Whip the cream until it forms firm peaks.
5. Gently incorporate the whipped cream with a spatula. Gradually incorporate the beaten egg whites, using a metal spoon, to maintain the mousse texture.
6. Pour into cups and refrigerate for 3 hours.