Combine cornmeal, water, and 1 cup milk in a medium saucepan; stir until blended. Cook over low heat until mixture is the consistency of mush. Remove from heat; add butter, baking powder, salt, and remaining milk. Beat egg yolks well; stir into warm mixture.
Beat egg whites (at room temperature) until stiff peaks form; fold into cornmeal mixture. Spoon into a greased 1 1/2-quart casserole. Bake at 350° for 45 minutes or until a knife inserted in center comes out clean. Serve immediately.