Step 1

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.

Step 2

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.