1. Preheat oven to 430ºF. Line baking sheet with parchment paper.
2. Whisk both flours (not the dusting flour), baking soda, and salt in a bowl.
3. Add buttermilk and stir until it's too hard to stir anymore.
4. Sprinkle 2 Tbsp flour onto work surface, turn dough onto surface, then sprinkle with a little more flour. Do not over-add flour. The goal is to use just enough to make the dough manageable.**
5. Gently knead no more than 8 times, then bring together into a ball and transfer to baking sheet. Pat into a disc about 1" thick.
6. Cut an X in the surface about ¼" deep using serrated knife.
7. Bake for 20 minutes. Reduce heat to 390ºF, and bake another 20 minutes, or until the top of the middle of the loaf sounds hollow when tapped.
8. Remove from oven, transfer to rack, and cool for at least 30 minutes before slicing.