Preheat oven to 400°. Whisk dry ingredients in a large bowl.
Divide butter between two 9- by 13-in. baking pans or ovenproof 12-in. frying pans and melt in oven, 4 minutes. Pour out all but a thin layer of butter into a small bowl to reserve; swirl remaining butter in pans to coat.
Whisk buttermilk and eggs in a medium bowl. Whisk in reserved melted butter. Pour buttermilk mixture into dry ingredients and stir until just combined.
Pour batter into pans and bake, swapping positions halfway through, until a toothpick inserted in center comes out clean, 25 minutes. Let cool.
*Find cornmeal at farmers' markets, healdsburgshed.com, and bobsredmill.com.
Make ahead: Up to 1 day, stored airtight.