In a medium saucepan over low heat, combine the cornmeal, grits, and water. Cook slowly, stirring frequently, until mixture is thick and smooth, about 25 to 30 minutes.
Beat in the butter, light cream, and salt.
Beat the egg yolks in a small bowl then beat them into the cornmeal mixture.
In a mixing bowl with an electric mixer, beat the egg whites until soft peaks form. Gently fold the beaten egg whites into the cornmeal mixture.
Pour the batter into a lightly buttered 8-inch square baking pan; bake in a preheated 375 F oven for about 30 to 40 minutes, or until lightly browned and puffy.
Serve the spoonbread immediately with butter.