Preheat oven to 400˚F. Place a cast iron skillet in the oven as it preheats.

In a large bowl, whisk together flour, salt, and baking soda. Set aside.

In a small bowl, whisk together melted butter and sugar. Add the mixture to the dry ingredients, stirring to combine. Note the mixture will still be dry at this point.

Pour in half of the buttermilk, mixing to incorporate. Add the remaining buttermik, mixing to combine. The dough should be shaggy and slightly moist. If the dough is very dry, add another teaspoon of buttermilk (I had to do this because I'm in a dry location!).

Pour the dough out onto a floured surface, kneading a few times to combine any dry bits that haven't been mixed in. Form into a ball, then flatten into a disc that is at least 2 inches thick.

Remove skillet from the oven and line with parchment paper. Place the dough disc in the center. Brush the top with buttermilk. Use a sharp knife to cut an "X" in the top that is 1/2 to 3/4-inch deep.

Bake for 40 minutes, until golden brown. Remove from oven and cover with aluminum foil for 10 minutes. Remove foil and let cool for 20 minutes before slicing.

Bread can be stored, covered, at room temperature for 3 days or in the refrigerator for up to 1 week.