Place the chocolate in a glass or metal bowl set over simmering water, and stir it until fully melted. (Note: the bowl should be wider than the pan of water so that the bottom of the bowl does not touch the water. You can also use a double boiler, if you have one.) Set the melted chocolate aside to cool to baby-food temperature.
Place the egg whites in a small mixing bowl with a pinch of cream of tartar and beat until the whites are stiff and glossy, but not dry. Set aside.
Place the cream and vanilla extract in a large mixing bowl (I use the stand mixer with the whisk attachment) and beat until the cream forms stiff peaks.
Pour the melted chocolate into the egg whites and fold them together until no streaks are left. Then, scrape this mixture into the whipped cream and very gently fold together, again to the point that no streaks are visible. (At this point it should look like chocolate mousse.)
Divide the mousse among four or six small wine glasses (see below) or decorative bowls; cover and refrigerate for at least two hours (or up to 2 days).