Chop the chocolate and melt it carefully in a bowl over warm water, then stir to smooth out. It will be well melted and smooth at around 140°F, or when it feels slightly warm to the touch. Warm the milk to about the same temperature and whisk it into the chocolate, just until it is smoothly mixed.

Whip the cream until it makes a slightly rounded shape when dropped from the beater; It should not stand in peaks, or the mousse will be grainy.

Warm the egg whites slightly by swirling them in a bowl over warm water or above a gas flame. Beat them with cream of tartar until they hold softly rounded peaks when the beater is withdrawn. Fold the whites into the chocolate mixture, being careful not to over mix. Then pour the mixture over the whipped cream, add the vanilla, and fold them together quickly. Taste and add more vanilla, if needed, then pour into sherbert cups or parfait glasses.

Chill well before serving.