Preheat oven to 425 degrees F. Line a baking sheet with parchment paper; set aside.
In a large bowl, whisk together the flours, baking soda and salt. Make a well in the center and pour in the buttermilk. Using a wooden spoon, stir the buttermilk into the flour mixture until a dough develops.
Turn the dough out onto a lightly floured work surface and, using floured hands, bring the dough together into a cohesive ball and pat into a round loaf about 7 inches in diameter.
Transfer the loaf to the prepared baking sheet. Cut a 1-inch deep "X" in the top of the loaf with a very sharp knife. Using a skewer, poke holes at 1-inch intervals all over the top of the dough, making sure to push through the whole way to the bottom.
Bake the bread for 30 minutes, then turn it upside-down on the baking sheet and bake for 10 additional minutes. Transfer the bread to a wire rack and cool completely before slicing, about 2 hours. The bread is best the day it is made, but it can be stored in a bread bag or wrapped in plastic wrap and kept at room temperature for up to 2 days.