With a fork prick sweet potatoes in several places and on a baking sheet bake in middle of oven until very tender, about 1 1/4 hours. When potatoes are cool enough to handle, remove skins and discard. In a food processor blend potatoes with vinegar and 1/4 teaspoon cinnamon until smooth. Transfer potatoes to a bowl and stir in sour cream until combined well. Season potatoes with salt and pepper. Sweet potatoes may be made 1 day ahead and chilled, covered. Reheat sweet potatoes, covered, in a 350°F. oven about 30 minutes.
Serve sweet potatoes sprinkled with remaining 1/4 teaspoon cinnamon.
