Heat milk and butter in a saucepan until small bubbles appear at the edge. Meanwhile, whisk the eggs in medium bowl. Very slowly stream about half of the hot milk mixture into the eggs, whisking constantly, then slowly stream the egg mixture back into the saucepan with the rest of the milk. (This prevents the eggs from curdling.) Cook, stirring constantly until the mixture reaches 165ºF, about 2 minutes.

Carefully transfer this custard base to the blender.  Add the chopped chocolate and blend on high speed for about 30 seconds.  Add the cream and blend for another 30 seconds. Then add the sugar, vanilla and salt and blend again for 30 seconds.

Pour the mousse into serving glasses and chill for about 2 hours, until set. Serve with whipped cream.