1 2-ounce block bittersweet (not unsweetened) chocolate, for chocolate shavings (optional) (see Tip)
Heat milk and orange zest in a small saucepan until steaming. Remove from the heat and let steep for 10 minutes. Discard the orange zest. Sprinkle gelatin over Grand Marnier in a small bowl; let stand until softened, about 1 minute.
Whisk together whole egg, ¼ cup brown sugar, cocoa and the infused milk in another 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. Set aside to cool to room temperature, about 30 minutes.
Bring about 1 inch of water to a simmer in a wide saucepan. 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 140°F. (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. With a rubber spatula, fold the chocolate mixture back into the remaining meringue until completely incorporated. Spoon the mousse into 6 dessert glasses and chill until set, about 3 hours.
If making chocolate shavings, place chocolate on wax paper and microwave, uncovered, at medium-low (30 percent) power for 15 seconds. Turn chocolate block over and microwave for 10 to 15 seconds longer, or just until the chocolate has softened slightly but has not started to melt. Use a vegetable peeler to shave off curls. (If the chocolate is too hard to shave easily, warm it again.) Garnish each mousse with chocolate shavings, if using, and serve.
Make Ahead Tip: The mousse can be stored, covered, in the refrigerator for up to 2 days.
Tip: To make chocolate shavings: Place a block of chocolate (2 ounces or larger) on wax paper and microwave on Defrost until slightly softened but not melted, 15 to 30 seconds. Use a swivel-bladed vegetable peeler to shave off curls. If the chocolate gets to hard to shave easily, warm it again.