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 the 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 the next 4 ingredients in a small bowl. Stir the cornmeal mixture into the squash mixture. Pour warm buttermilk over the squash mixture; whisk until smooth. Let stand 15 minutes or until lukewarm.
Brush a 2 1/2 to 3 quart baking dish or 12-inch cast-iron skillet with 1 tablespoon melted butter; stir remaining melted butter into the squash mixture.
Beat the egg whites at high speed with an electric mixer until stiff peaks form. Carefully fold into the squash mixture. Pour the mixture into the prepared baking dish.
Bake at 350° for 30 to 35 minutes or until the top is golden and a wooden pick inserted in the center comes out clean.