Preheat oven to 425°F (220°C). Lightly dust a baking sheet with flour or line it with parchment paper.
In a large bowl, whisk together flour, baking soda, salt, and sugar (if using).
Add cold butter (if using) and rub it into the flour with your fingers until the mixture resembles coarse crumbs.
Make a well in the center and pour in the buttermilk. Stir with a wooden spoon until a shaggy dough forms (do not overmix).
Turn the dough onto a lightly floured surface. Gently shape it into a round loaf (~6–7 inches wide).
Place it on the baking sheet. Use a sharp knife to score a deep "X" on top (helps with even baking).
Sprinkle with oats or flour for texture.
Bake for 35–40 minutes until golden brown and the bottom sounds hollow when tapped.
Cool on a wire rack for 10 minutes before slicing.
