Remove giblets from hens; discard. Rinse hens with cold water, and pat dry. Split hens down backs, and open flat; lay hens, breast side down, on a lightly greased rack in a shallow roasting pan. Rub with butter. Sprinkle with salt and pepper.
Broil hens 5 inches from heating element 10 minutes on each side or until drumsticks move easily. Transfer hens to a warm serving platter; garnish with watercress. Serve immediately.