Boil water, then add peeled and washed potatoes, cut into small pieces. Cook for about 20 minutes, until soft. Trim the bottoms off the watercress and discard. Add watercress to potatoes in water and cook for another 10 minutes. Drain and process in a blender or food processor until creamy. Add butter and pulse to blend. Add salt and pepper. Just before serving, reheat just to a simmer. Serve with croutons.