Preheat oven to 450 degrees. With a fork, pierce 4 medium sweet potatoes all over and rub with 1 tablespoon olive oil. Bake on a rimmed baking sheet lined with foil, until a paring knife inserted in the center meets no resistance, about 45 minutes. Cut a deep cross into the middle of each sweet potato and push the ends together to open. With a fork, loosen and lightly mash the insides of the sweet potatoes. Top each with a tablespoon butter.