1. Chop the chocolate and place it in a mixing bowl.
2. Bring the milk and cream to a boil, then pour over the chocolate.
3. Mix until completely melted.
4. Add the butter, mix again, then let cool to 40°C.
5. Separate the egg whites from the yolks.
6. Beat the egg whites until stiff, then fold in the sugar.
7. Gently incorporate the yolks, then fold everything into the ganache in several additions.
8. Pour the mousse into a bowl and let it rest in the refrigerator overnight.
9. Just before serving, grate some dark chocolate over the mousse.