In a bowl, mix flour, polenta, sugar, baking powder, and salt. In another bowl, beat eggs to blend with buttermilk and 1/4 cup butter; pour into flour mixture. Stir batter until evenly moistened.
Scrape batter into a buttered 8-inch square pan and spread level.
Bake in a 400° oven until cornbread is browned, springs back when lightly pressed in the center, and begins to pull from pan sides, 20 to 25 minutes. Use hot or cool.