For the ideal texture, we recommend serving this immediately once baked.
Preheat oven to 350°. Cook buttermilk in a heavy saucepan over medium-high heat, stirring often, 4 to 6 minutes or until bubbles appear around edges (do not boil); remove from heat. (Mixture may curdle.)
Lightly beat egg yolks in a large bowl; stir in squash and cheese. Combine cornmeal and next 4 ingredients in a small bowl. Stir cornmeal mixture into squash mixture. Pour warm buttermilk over squash mixture; whisk until smooth. Let stand 15 minutes or until lukewarm.
Brush a 2 1/2- to 3-qt. baking dish or 12-inch cast-iron skillet with 1 Tbsp. melted butter; stir remaining melted butter into squash mixture.
Beat egg whites at high speed with an electric mixer until stiff peaks form. Carefully fold into squash mixture. Pour mixture into prepared baking dish.
Bake at 350° for 30 to 35 minutes or until top is golden and a wooden pick inserted in center comes out clean.
Note: We tested with Bird's Eye Frozen Butternut Squash. Buy 2 (12-oz.) packages to measure 2 cups.