Blend the chocolate powder with the butter previously melted.
In a separate bowl stir the egg yolk till having a foamy mass and mix together with the chocolate powder and butter.
Whip the cream and add to the former mixture.
Beat the egg white constantly until stiff peaks form.
Add to the mass slowly to get the mousse consistency and pass to the fridge.

Result:
As it is an artisanal mousse we obtain less consistency in the final product than the industrial mousses although more density as the aeration is less efficient.
White chocolate mousse is not a frequent dessert but it is the election in sophisticated dishes of combined tastes with strong flavoured fruits, mint or plain chocolate, with strong cocoa content.
Served alone can be decorated with a chocolate sauce and some granules of our granulated plain chocolate.