In a large saucepan, heat 3 cups milk over medium heat until bubbles form around sides of pan.
Meanwhile, in a small bowl, combine the cornmeal, sugar, salt and remaining milk until smooth. Slowly whisk cornmeal mixture into hot milk. Cook and stir until mixture comes to a boil. Reduce heat; simmer for 5 minutes, stirring constantly.
Remove from the heat. Sprinkle baking powder over the cornmeal mixture, then stir it in with the butter. In a small bowl, beat egg yolks; stir in a small amount of hot cornmeal mixture. Return all to the pan and mix well.
In a small bowl, beat egg whites until stiff peaks form. Fold a fourth of the egg whites into the cornmeal mixture. Fold in remaining egg whites until blended.
Transfer to a greased 2-1/2-qt. baking dish. Bake, uncovered, at 350° for 40-45 minutes or until puffed and golden brown. Serve immediately.