How to Make It
Preheat oven to 375°F with oven rack in lower third of oven. Stir together granulated sugar and flour in a small bowl; set aside. Cut a 24- x 12-inch piece of parchment paper; fold lengthwise 3 times (letter- style). Wrap paper around outside top of a 1-quart soufflé dish to form a collar extending 2 inches above rim; secure tightly with string or tape. Rub inside of soufflé dish and parchment collar with butter. Dust with sugar mixture, shaking out excess. Chill dish at least 15 minutes.
Stir together crème pâtissière, Grand Marnier, and orange zest in a large bowl. Beat egg whites in bowl of a heavy-duty stand mixer on medium speed until glossy and stiff peaks form, about 4 minutes. Whisk about one-third of egg whites into crème pâtissière mixture until well incorporated. Gently fold in remaining egg whites until just incorporated. Pour mixture into prepared dish. Place dish on a rimmed baking sheet, and bake in preheated oven until soufflé is puffed and golden brown, 40 to 45 minutes. Sprinkle with powdered sugar. Serve immediately.