1. Break the chocolate into small pieces in a bowl. Melt the chocolate in a double boiler.
2. Add the hazelnut paste and melt, stirring with a spoon. Set aside off the heat. Add the butter in small cubes.
3. Separate the egg whites from the yolks. Beat the egg whites until stiff.
4. Add the egg yolks to the melted chocolate. Mix to obtain a smooth paste.
5. With a spatula, gently incorporate the beaten egg whites to retain the mousse texture.
6. Pour the mousse into ramekins and refrigerate for 1 hour before serving.