5 tablespoons unsalted butter, plus more for the pan
3/4 cup all-purpose flour
3/4 cup yellow cornmeal
1 1/2 teaspoons baking powder
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
1 cup minus 1 tablespoon milk
1 tablespoon vinegar
2 large eggs
Make buttermilk – combine the milk and vinegar, let set for at least 10 minutes.
Preheat the oven to 425 degrees F and position a rack in the middle.
Butter an 8- inch-square baking pan.
Melt the butter in the microwave in a microwave-safe dish in three 15-second intervals on high or in a small pan on the stove. Set it aside to cool.
In a bowl, whisk together the flour, cornmeal, baking powder, baking soda and salt.
In a large bowl, whisk together the buttermilk and the eggs. Add the melted butter.
Add the flour-cornmeal mixture and stir just until combined.
Pour the batter into the pan.
Bake until the cornbread just begins to brown and a toothpick inserted in the center comes out clean, 18 to 23 minutes.
Cool for about 10 minutes before serving.