Preheat oven to 350. Scrub sweet potatoes, prick and oil skins. Place on roasting pan sprayed with cooking spray. Cut off top of the pod of garlic. Place in garlic roaster or on foil. Pour olive oil over the garlic and cover roaster or wrap in foil. Place in oven with sweet potatoes. Cook for about 1 hour. Remove from oven. Skin sweet potatoes and push garlic cloves out of their skins. Cut up potatoes and place in bowl with garlic and butter. Mash with potato masher until fluffy. Add salt and pepper.