Roasted sweet potatoes are simple and delicious, but to make your sweet potatoes even better this Thanksgiving, roast them with the skin on, mash them, then bake them again. Your mashed sweet potatoes will be smooth and have a citrusy kick from the orange juice and sherry vinegar.
Preheat the oven to 400 degrees F.
Put the potatoes right on the oven rack. No foil. No baking sheet. This will allow the oven heat to circulate more freely around the potatoes as they cook. Put a layer of foil on the floor of the oven to avoid messy cleanup. Roast until the potatoes are completely yielding in the center when pierced with the tip of a knife, 1 hour to 1 hour 30 minutes, depending on their size. Remove from the oven and lower the oven temperature to 300 degrees F.
Use a sharp knife to cut lengthwise down the middle of each sweet potato. Scoop out the flesh with a tablespoon, leaving the skin behind. Transfer the flesh to a food processor and pulse to blend until smooth. Do not overblend or it will make the potatoes gummy. Put the potato in a medium ovenproof dish. Spread it out in an even layer.
Once the oven registers 300 degrees F, bake the sweet potatoes for 10 minutes and then give the flesh a stir again. Bake for an additional 10 minutes. Remove from the oven. Transfer the sweet potato flesh to a medium bowl and season with salt. Stir in some of the orange juice, the sherry vinegar, and butter, if using. Taste for seasoning. Add more orange juice if needed.