1 1/3 cup yellow cornmeal
1 3/4 cups flour
2 tablespoons sugar
1 1/2 teaspoons baking powder
1/2 teaspoon baking soda
1 1/4 teaspoons kosher salt
1 3/4 cups buttermilk
1 egg plus 1 large yolk
4 tablespoons unsalted butter, cut into 4 pieces
Preheat oven to 425F
In a medium bowl, combine cornmeal, flour, sugar, baking powder, baking soda, and salt.
In a large measuring cup, whisk together buttermilk, egg, and egg yolk.
Pour the buttermilk mixture in with the cornmeal mixture and stir until well combined.
Add the butter to a cast iron skillet (at least 10 inches wide) and place in the oven until the butter is melted (3 to 5 minutes).
Remove the skillet from the oven, and swirl the butter around to make sure the skillet bottom and sides are well coated.
Pour the batter into the skillet, and smooth out the top with a rubber spatula.
Bake for 20 to 25 minutes, or until a toothpick or skewer inserted into the center comes out clean.