Bring a few inches of water to a boil in the lower half of a double boiler, then lower the heat. Bring the quart of water to a boil in the top of the double boiler, placed directly over the heat. Add the salt and whisk in the cornmeal. Cook, stirring constantly, until the consistency is smooth, after several minutes, then set the pan over the simmering water and cover. Cook, giving it a stir every 20 minutes or so, until tender and cooked through, about 1 1/4 hours. Check now and then to make sure there’s ample water in the bottom of the double boiler. Just before serving, stir in the butter as desired and the cheese, if using.