Our mousse reformation, with only 248 calories and 6 grams of fat per serving, as compared to 485 calories and 40 grams of fat in the original recipe, doesn't require a warning from the Surgeon General.
Sprinkle gelatin over liqueur (or rum or coffee) in a small bowl; let stand until softened, about 1 minute.
Whisk together milk, whole egg, ¼ cup of the brown sugar and cocoa in a medium saucepan until smooth. Cook over low heat, whisking constantly, until thickened, about 5 minutes. Remove from the heat and add the softened gelatin mixture, stirring until the gelatin has dissolved. Add chocolate and vanilla; stir until the chocolate has melted. Let cool to room temperature, about 30 minutes.
Bring 1-inch of water in a wide saucepan to a bare simmer. Combine egg whites, cream of tartar, water and the remaining ¾ cup brown sugar in a heatproof bowl large enough to fit over the saucepan. Set the bowl over the barely simmering water and beat with an electric mixer at low speed, moving the beaters around constantly, until an instant-read thermometer registers 140F. (This will take 3 to 5 minutes.)
Increase the mixer speed to high and continue beating over the heat for a full 3½ minutes. Remove the bowl from the heat and beat the meringue until cool, 4 to 5 minutes longer.
Whisk one-fourth of the meringue into the chocolate mixture until smooth. Fold the chocolate mixture back into the remaining meringue with a rubber spatula until completely incorporated. Spoon the mousse into 6 dessert glasses, and chill until set, about 3 hours.