• 4 Tbsp (1/2 stick) unsalted butter, melted, plus more for baking dish
• 4 pounds sweet potatoes
• 2 Tbsp dark brown sugar
• 1/4 tsp freshly grated nutmeg
• 1/8 tsp cayenne pepper
• 3 large eggs, plus 4 large egg whites
• 1 1/2 tsp pure vanilla extract, divided
• Kosher salt and freshly ground black pepper
• 2/3 C granulated sugar
• 1/2 tsp cream of tartar
Preheat oven to 375 degrees. Butter a 3-quart, broiler-safe baking dish. Roast sweet potatoes on a baking sheet until tender, 60 to 70 minutes. Cut a slit in the top to release steam and let cool for 15 minutes. Scoop out sweet potato flesh (discard skins). Transfer to a food processor and process until completely smooth, 30 seconds. Add butter, brown sugar, nutmeg, cayenne pepper, whole eggs, and 1 tsp vanilla. Pulse until fully incorporated, three to four times. Season with salt and black pepper; transfer to the prepared dish. Bake until sweet potatoes are set, 20 to 25 minutes. Let stand for at least 10 minutes. Whisk together egg whites, granulated sugar and cream of tartar in a bowl. Set the bowl over (but not in) a saucepan of simmering water and cook, whisking constantly, until sugar is dissolved and whites are very warm to the touch, 3 to 4 minutes; remove from heat. Beat with an electric mixer on low speed, gradually increasing speed to high, until glossy and soft peaks form, 4 to 6 minutes. Beat in the remaining vanilla. Heat the broiler with rack in the middle position. Spoon meringue over cooled sweet potatoes. Broil until meringue is browned, 30 seconds to 1 minute.
