3lbssweet potatoes, peeled and cut into 1 1/2-inch pieces
4tablespoonsunsalted butter, cut into 6 pieces
Adjust oven rack to middle position and heat oven to 400 degrees. Mix 3 tablespoons sugar and 1/2 teaspoon orange zest together in small bowl until thoroughly combined; set aside.
Lay two 24 by 12-inch sheets of heavy-duty aluminum foil perpendicular to each other inside rimmed baking sheet. Place sweet potatoes in center of foil and sprinkle with remaining 1 tablespoon sugar. Fold opposite sides of foil toward each other and crimp edges to seal tightly. Put baking sheet in oven and bake until sweet potatoes are tender, about 60 minutes. Remove baking sheet from oven and heat broiler.
Carefully open 1 end of foil pouch, taking care to avoid escaping steam, and pour potatoes and accumulated liquid into food processor. Add butter, cream, orange juice, salt, cinnamon, remaining 1/2 teaspoon orange zest, pepper, and cayenne and process until completely smooth, 30 to 60 seconds, scraping down bowl as needed.
Transfer potato puree to 2-quart soufflé dish and sprinkle evenly with reserved sugar-zest mixture. Broil sweet potatoes until topping is lightly browned and bubbling, 2 to 4 minutes. Serve.
TO MAKE AHEAD: Processed potatoes can be refrigerated without topping for up to 2 days. To serve, cover dish with aluminum foil and bake on middle rack of 400-degree oven until hot throughout, 30 to 40 minutes. Remove foil, add brown sugar topping, and broil as directed in step 4.