Preheat oven to 375 degrees. Butter 8 cup souffle dish. Bring milk to boil in heavy large saucepan. Reduce heat to medium. Gradually mix in cornmeal and salt. Stir until mixture becomes very thick, about 1 minute. Add butter; stir until melted. Remove pan from heat. Stir in 1/2 cup sugar and buttermilk. Whisk yolks in large bowl to blend. Gradually whisk in cornmeal mixture. Using electric mixer, beat egg whites in medium bowl until frothy. Add remaining 1/4 cup sugar and beat until soft peaks form. Gently fold whites into cornmeal mixture.
Transfer mixture to prepared dish. Bake until mixture puffs and is golden brown. Sprinkle with powdered sugar and serve hot.
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