1. For the white chocolate mousse; open the vanilla pod with a knife and scrape out the black seeds. Add these seeds to the cream. Heat the cream over very low heat in a small non-stick saucepan. Incorporate the white chocolate pieces, a little at a time. Make sure the mixture is smooth and homogeneous. Remove the saucepan from the heat, let it cool, then place the mixture in the refrigerator for at least 2 hours. Remove the mousse from the refrigerator and whip it with an electric mixer. Divide the mousse among the glasses, tilting them to fill them halfway and at an angle. Refrigerate to set for 20 minutes.
2. For the dark chocolate mousse, break the chocolate into small pieces in a bowl. Melt it in a bain-marie. Set aside off the heat and add the butter in small cubes.
3. Separate the egg whites from the yolks. Beat the egg whites until stiff peaks form and set aside. Add the egg yolks to the melted chocolate, mix to obtain a smooth mixture.
4. Incorporate the egg whites into the chocolate mixture with a spatula, a little at a time, and gently, so as not to lose the mousse texture.
5. Remove the glasses from the refrigerator and fill the remaining space with the dark chocolate mousse.
6. Refrigerate for another 30 minutes before serving.