Heat the oven: Preheat to 425°F (220°C).
Line a baking sheet with parchment or lightly flour a cast-iron skillet.
Mix dry ingredients: In a large bowl, whisk flour, baking soda, salt, and sugar (if using) until evenly combined.
Cut in the butter: Add the cold butter. Rub it into the flour with your fingertips until it looks like coarse crumbs. If adding raisins, seeds, or oats, stir them in now.
Add buttermilk: Make a well in the center.
Pour in 1 3/4 cups buttermilk and stir with a fork until a shaggy dough forms. If it’s very dry, add another tablespoon or two of buttermilk.
Bring it together: Turn the dough onto a lightly floured surface. Gently knead 5–8 times, just until it holds together.
The dough should be soft and slightly sticky, not smooth.
Shape the loaf: Form a round about 7 inches wide and 1 1/2–2 inches tall. Place it on the prepared sheet or skillet. Dust lightly with flour.
Score the cross: Use a sharp knife to cut a deep cross on top, about 1/2 inch deep.
This helps it cook evenly and open up as it bakes.
Bake: Bake for 15 minutes at 425°F (220°C). Reduce heat to 400°F (200°C) and bake 20–25 minutes more. It’s done when the crust is deep golden and the bottom sounds hollow when tapped, or when an instant-read thermometer reads about 200–205°F (93–96°C) in the center.
Cool briefly: Transfer to a rack and let cool at least 30 minutes before slicing.
Cutting too soon can make the center gummy.
Serve: Slice thick and enjoy warm with butter, jam, honey, or alongside soup and stews.