Sift the flours, soda and salt into a bowl. Make a well in the center and add the butter and buttermilk. Working outwards from the center, stir with a fork until a soft dough is formed.
With floured hands gather the dough into a ball. Transfer to a floured surface and knead for 3 minutes. Shape the dough into a large round. Place on the baking sheet and cut a cross on the top with a sharp knife. Dust with flour and bake until browned, 40-50 minutes.
Transfer to a rack to cool.
