Combine barley flour, oat flour, baking soda, and salt in a bowl. Make a well in the center and pour in the buttermilk. Mix until a soft dough forms. It should not be sticky but it should be pliable. Add a little more buttermilk a tablespoon at a time if the dough feels dry or crumbly.
Turn the dough onto a lightly floured surface and bring it together gently. Do not overwork it. Shape it into a round about 1 inch thick and roughly the diameter of your pan. Score the top into four sections with a knife, cutting about halfway through. Rest for 10 minutes.
Melt butter in a cast iron pan over medium-low heat. Place the bannock in the pan. Cover with a lid and cook for 8 to 10 minutes until the bottom is deep golden. Flip carefully and cook another 8 to 10 minutes covered until cooked through. A skewer inserted in the center should come out clean.
Rest for 5 minutes before breaking apart at the scored sections. Serve warm.