For the butter: Place the butter in a small bowl and mix with a fork until light. If it's too cold to lighten, microwave for 10 seconds. Mix in the remaining ingredients.
For the sweet potatoes: Preheat the oven to 425°F. Line a baking sheet with foil. Pierce each potato several times with a fork and place on the pan. Rub each potato with olive oil and then sprinkle with salt and pepper. Roast until soft when pressed, 45 minutes to 1 hour, depending on the size of the potatoes. Cut a slit in the top of each and squeeze gently. Serve with the butter.