In a saucepan over high heat, bring the water and salt to a boil. Reduce the heat to medium and add the cornmeal in a stream whisking constantly. Cook over medium-low heat, stirring frequently, for 15 minutes, or until the mixture is thick. Sitr in the butter and season with pepper, then pour the polenta into a buttered 9-inch-square shallow baking pan to cool. Cover and chill for at least 30 minutes.