Preheat the oven to 375°F (190°C). Pat the duck dry with paper towels so the skin roasts crispy.
Season the duck inside and out with salt and black pepper. Place it breast-side up on a rack in a roasting pan.
Roast the duck for about 60–70 minutes, occasionally spooning rendered fat over the skin until golden brown and crisp.
While the duck roasts, combine orange juice, orange zest, honey, brown sugar, soy sauce, and rice vinegar in a small saucepan.
Bring the sauce to a gentle simmer over medium heat, stirring occasionally until slightly reduced and fragrant.
Mix cornstarch with water to create a slurry, then whisk it into the simmering sauce until it thickens into a glossy glaze.
Stir in butter for a smooth finish and remove from heat once the sauce coats the back of a spoon.
Carve the roasted duck into portions and drizzle generously with the orange sauce before serving.
