Preheat oven to 325°F. Lightly butter six 3/4-cup soufflé dishes or custard cups. Beat egg yolks and 1/2 cup sugar in medium bowl to blend. Bring cream, milk and orange peel to simmer in medium saucepan over medium-high heat. Gradually whisk hot cream mixture into yolk mixture. Stir in Grand Marnier.
Divide custard among prepared dishes. Set dishes in large baking pan. Add enough hot water to pan to come halfway up sides of dishes. Bake custards until just set in center, about 40 minutes. Remove from water. Cool, then cover and refrigerate overnight.
Preheat broiler. Arrange soufflé dishes on baking sheet. Sprinkle 1 teaspoon sugar over each. Broil until sugar browns, rotating baking sheet for even browning and watching closely to avoid burning, about 2 minutes. Refrigerate 1 hour. (Can be made 6 hours ahead. Keep refrigerated.)