Preheat the oven to 425. With a pastry brush coat a baking sheet evenly with the tablespoon of softened butter. Sift the flour, soda and salt together into a deep mixing bowl. Gradually add 1 cup of the buttermilk, beating constantly with a large spoon until the dough is firm enough to be gathered into a ball . If the dough crumbles, beat up to 1/2 cup more buttermilk into it by the tablespoon until the particles adhere.
Place the dough on a lightly floured board and pat and shape it into a flat circular loaf about 8" in diameter and 1 1/2 " thick. Set he loaf on the baking sheet. Then with the tip of a small knife cut a 1/2" deep X into the dough,dividing the top of the loaf into quarters.
Bake the bread in the middle of the oven for about 45 minutes or until the top is golden brown.
There are a couple of variations on this. You can add 1 cup currants or raisins with the flour. Also you can sprinkle the top of the loaf with caraway seeds.