Heat oven to 375ºF. Pierce sweet potatoes with fork to allow steam to escape. Bake about 45 minutes or until tender. Cut thin lengthwise slice from each potato; carefully scoop out inside of potato, leaving a thin shell. Mash potatoes in large bowl until no lumps remain. Beat in sour cream and milk. Beat in brown sugar, butter and salt until potatoes are light and fluffy. Place shells in ungreased rectangular baking dish, 13x9x2 inches. Fill shells with potato mixture. Cover and refrigerate no longer than 24 hours. Heat oven to 400ºF. Bake uncovered about 25 minutes or until potato mixture is golden brown. (If baking filled shells immediately after mashing potatoes, bake about 20 minutes.)