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.
Place potatoes in a large bowl; stir in cornmeal mixture. Stir in egg yolks and baking powder, stirring until well blended.
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.
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.