Preheat oven to 400°F (200°C). Place an 8-inch cast-iron skillet in oven while oven preheats.

In a medium bowl, whisk together fl our, cornmeal, sugar, baking powder, salt, and baking soda.

In another medium bowl, whisk together buttermilk, mayonnaise, eggs, and melted butter. Whisk buttermilk mixture into fl our mixture just until combined.

Working quickly but carefully, add butter to skillet, swirling skillet until butter is melted and skillet is evenly coated. Pour batter into skillet.

Bake until edges are browned, top is crisp, and an instant-read thermometer inserted in center registers 205°F (96°C) to 210°F (99°C), 20 to 25 minutes, covering with foil to prevent excess browning, if necessary. Let cool for 5 to 10 minutes. Best served hot. Store in an airtight container for up to 2 days.