1. Break the chocolate into small pieces in a bowl. Melt it in a double boiler. Set aside off the heat. Add the butter in small cubes and mix.
2. Separate the egg whites from the yolks. Beat the egg whites until stiff peaks form. Add the egg yolks to the melted chocolate, whisking to obtain a smooth mixture.
3. With a spatula, gently fold in the beaten egg whites to retain the mousse texture.
4. Pour half of the chocolate mousse into the glasses.
5. Add the peanut butter to the remaining mousse and mix. Pour the preparation over the chocolate mousse.
6. Refrigerate for 3 hours before serving.