1. Break the chocolate into small pieces in a bowl. Melt the chocolate in a double boiler.
2. Off the heat, add the butter. Separate the egg whites from the yolks.
3. Beat the egg whites until stiff peaks form. Add the egg yolks to the melted chocolate, mix to obtain a smooth mixture.
4. Incorporate the stiff egg whites into the mixture with a spatula, little by little and gently, to avoid losing the mousse texture.