Preheat oven to 220°C/430°F (200°C fan).
Line tray with baking paper.
Whisk both flours (not Extra Flour), baking soda and salt in a bowl.
Add buttermilk, stir until it's too hard to stir anymore.
Sprinkle 2 tbsp Extra Flour onto work surface, scrape out dough, sprinkle with more flour.
Gently knead no more than 8 times, bring together into a ball. (Note 4)
Transfer to tray, pat into 2.5cm/1" thick disc.
Cut cross on surface 1cm / 0.3" deep using serrated knife.
Bake 20 minutes. Turn oven down to 200°C/390°F (180°C fan).
Bake further 20 minutes, or until the base sounds hollow when tapped in the middle.
Transfer to rack and cool for at least 30 minutes before slicing.
