Preheat oven to 450 degrees F. Coat a baking sheet with cooking spray and sprinkle with a little flour.
Whisk whole-wheat flour, all-purpose flour, baking soda and salt in a large bowl. Make a well in the center and pour in buttermilk. Using one hand, stir in full circles (starting in the center of the bowl working toward the outside of the bowl) until all the flour is incorporated. The dough should be soft but not too wet and sticky. When it all comes together, in a matter of seconds, turn it out onto a well-floured surface. Clean dough off your hand.
Pat and roll the dough gently with floury hands, just enough to tidy it up and give it a round shape. Flip over and flatten slightly to about 2 inches. Transfer the loaf to the prepared baking sheet. Mark with a deep cross using a serrated knife and prick each of the four quadrants.
Bake the bread for 20 minutes. Reduce oven temperature to 400° and continue to bake until the loaf is brown on top and sounds hollow when tapped, 30 to 35 minutes more. Transfer the loaf to a wire rack and let cool for about 30 minutes.