Bring water and salt to a boil in a large saucepan. Gradually whisk in the cornmeal in a slow steady stream. Lower heat and continue to whisk until the polenta is thick and smooth, about 20 minutes. Add the cream and butter and continue to stir until incorporated. Remove from heat, fold in Parmesan and black pepper. Pour the polenta into a buttered 9 by 13-inch shallow baking dish and spread evenly with a spatula. Cover and chill a few hours. Cut the polenta into squares. Brush both sides with olive oil and transfer to a hot grill. Grill on both sides until golden brown.