Preheat oven to 425°F.
In a medium bowl, whisk together the flour, baking powder, and salt. With a pastry cutter, cut in the cold pats of butter.
Pour the buttermilk into a measuring cup and stir in the baking soda.
Make a hollow in the dry ingredients and pour in the buttermilk. Stir until just blended.
Place dough on a lightly floured board. Turn dough to lightly cover with flour. Pat dough to ½-inch thickness. Cut with biscuit cutters and place on a baking sheet. Bake for 12 to 15 minutes in the preheated oven. Remove from oven and lightly brush with softened butter.