2Place both flours, the salt and baking soda in a large bowl. Whisk to combine.
3Pour the buttermilk into the flour mixture, one half at a time. After each addition, mix the buttermilk gently into the flour mixture with your hands, gathering the mix from the sides and pushing towards the middle until the mixture just comes together.
4Turn the dough out on to a lightly floured surface and form it into a ball, using a folding motion (rather than kneading) by turning it over on itself.
5Cut the dough in half using a bench scraper or knife and form into two balls. Cut deep crosses in the center of each ball and push the sides together to close the cut areas slightly. Place the dough balls on a floured sheet pan and sprinkle more flour on the tops.
6Bake the breads for 15 minutes at 450 degrees F, then reduce the heat to 400 degrees F and bake until the bread is golden, about 30 minutes more.