Chocolatier, Patrice Chapon stated the best chocolate to use for his mousse is 100% Venezuela. Using the strongest cacao content, this mousse is slightly dry in texture and intense. After chilling for at least 6 hours, the bitterness mellows but the best way to serve is simply on its own to appreciate the flavours.
Heat the milk until it boils. Grate the chocolate in a large bowl.
Pour the hot milk over the grated chocolate and stir gently until well mixed using a wooden spoon. Add the yolk* and continue to stir until the mixture is brilliant.
In a separate bowl, whip up the egg whites using an electric whisk, adding half of the sugar at first and then at the end when they are whipped and fluffy (but not firm).
Gradually incorporate the egg whites, folding it in delicately until all mixed together and smooth.
Either keep it in the bowl or transfer to individual serving bowls (I would suggest little ones here, as this mousse is so intense!)
Refrigerate at least 6 hours or, at best, overnight (covered with cling film) to enjoy next day.
Notes
Serve chilled with ground pistachios, almond tuile cookies or salted caramel macarons.*Yolk: for those of you worried about the quality of your egg yolk to be eaten raw, then this can be omitted.