Preheat oven to 375°F (190°C). Prepare 4 pieces of aluminum foil.
Scrub sweet potatoes well. Pat dry with paper towels. Prick each sweet potato evenly with a fork.
Place each sweet potato on a foil. Drizzle with a teaspoon of vegetable oil and rub the oil all over the sweet potato.
Wrap loosely with aluminum foil, making sure it is sealed. Place on a baking tray and into the oven. Roast for 60 minutes.
Open the foil and continue to roast for another 30 minutes. Insert a knife or fork into the center of each sweet potato. They should feel quite soft. If not, return to oven and continue roasting until sweet potatoes are soft.
Serve with butter and cinnamon sugar if desired.