Rub Cornish hens with butter; sprinkle salt and pepper inside cavity and over skin. Place on a rack in a shallow roasting pan. Bake, uncovered, at 350° for 30 minutes.
Meanwhile, combine the remaining ingredients in a saucepan. Bring to a boil. Reduce heat; simmer, uncovered, for 10 minutes. Baste hens with sauce. Bake 45 minutes longer or until juices run clear and a thermometer reads 180°, basting frequently.