Preheat oven to 350 degrees. Butter and dust a 1-1/2 quart souffle dish with sugar. Put egg whites in mixer bowl with cream of tartar. In a medium saucepan, stir together sugar, cocoa, flour, and salt. Stir in milk and Grand Marnier and cook over medium heat, stirring constantly until mixture boils and is smooth and thickened. Stir in the vanilla and orange peel. Set aside. Beat the egg whites only until they cling to the side of the mixing bowl when you tilt it. Stir the yolks into the sauce. Mix about a quarter of the whites right into the sauce in the pan, then empty the sauce into the whites in the mixing bowl. Gingerly but thoroughly fold the sauce into the whites. Then pour the mixture into the prepared souffle dish. Bake for about 30 to 40 minutes. Don't open oven door for at least 30 minutes. Serve immediately.