Melt the chocolate in a microwave oven in 30-second bursts, stirring it between each until it's completely melted and smooth. (This can also be done in a bowl over a pan of boiling water.)
In another bowl, whip the egg yolks until they become distinctly lighter in color. Combine the coffee and hot milk, and add it slowly to the egg yolks, whisking as you go.
Add the chocolate slowly to the egg-and-coffee mixture, and whisk well until the mixture is just barely warm and well-blended.
Beat the egg whites until soft peaks form, then add the sugar and vanilla. Continue beating until stiff. With a rubber spatula, blend the egg whites into the chocolate mixture. Do this gently, and don't worry that it will not be an absolutely uniform mixture.
Whip the whipping cream in a metal bowl. Remember that cream whips best when cold, and that if you overwhip it will break into butter and buttermilk.
If the chocolate mixture is still warm, let it continue to cool to room temperature. Then fold in the whipped cream with the rubber spatula or wooden spoon. Do this gently, and keep at it until you have a uniform texture.
Spoon the mousse into serving dishes, or pipe it in with a pastry bag for a more elegant presentation. You can also top it with shaved chocolate or a strawberry.