Preheat oven to 375°F and spray a 12 inch cast iron skillet with non-stick baking spray.
In a small bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.
2 cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon kosher salt
In a stand mixer fitted with the paddle attachment, cream the butter and sugar together on medium speed until light and fluffy, about 3 minutes.
8 tablespoon butter, 1¼ cups sugar
Add the eggs and egg yolk one at a time, beating for 1 minute between each addition and scraping the sides of the bowl occasionally. Add the vanilla and mix until just combined.
2 eggs + 1 egg yolk, 2 teaspoon vanilla
Add half the dry mixture and mix on low speed until just combined. Add the buttermilk and mix until mostly combined. Then add remaining dry ingredients and mix until just combined. The batter will be thick.
1 cup buttermilk
Pour the batter into the prepared skillet and spread evenly to the edges. Bake for 25-35 minutes until golden brown and a toothpick inserted in the middle comes out clean. Don't overbake.