1. Preheat oven to 350°. Bring first 5 ingredients to a simmer in a 3-qt. saucepan over medium heat. Whisk cornmeal into milk mixture in a slow, steady stream. Cook, whisking constantly, 2 to 3 minutes or until mixture thickens and pulls away from bottom of pan. Remove from heat, and stir in butter. Cool 10 minutes.
2. Place potatoes in a large bowl; stir in cornmeal mixture. Stir in egg yolks and baking powder, stirring until well blended.
3. Beat egg whites at high speed with an electric mixer until soft peaks form; fold into potato mixture. Spoon batter into a well-buttered 3-qt. baking dish.
4. Bake at 350° for 40 to 45 minutes or until golden brown and puffy. (Edges will be firm and center will still be slightly soft.) Cool 10 minutes on a wire rack before serving.