Put oven rack in middle position and preheat oven to 375 degrees F. Generously butter a 2 1/2 to 3-quart souffle dish (3 to 4-inches deep) or other deep baking dish.
Heat the milk in a 2-quart heavy saucepan over moderate heat until hot but not boiling. Add the cornmeal in a stream, whisking, and cook, whisking, until very thick and smooth, about 2 minutes. Transfer to a large bowl and stir in the butter, salt, baking powder, and yolks. Cool to lukewarm.
Beat the egg whites with a pinch of salt in clean bowl using an electric mixer until they just hold stiff peaks. Stir 1/4 of the whites into the cornmeal mixture to lighten, then fold in remaining whites gently but thoroughly.
Spoon batter into the prepared baking dish and bake until golden brown and puffed, 45 to 50 minutes. Serve immediately.