Heat the oven to 425°F (220°C). Line a baking sheet with parchment, or lightly grease a cast-iron skillet or round baking pan.

Whisk together the flour, baking soda, and salt in a large bowl. Add the sugar if using, then stir in the raisins or currants and rub in the cold butter if making a richer loaf.

Make a well in the center and pour in most of the buttermilk. Mix with a fork or sturdy spatula just until a shaggy, soft dough forms, adding the remaining buttermilk only if the dough seems dry.

Turn the dough onto a lightly floured surface and knead very briefly for about 20-30 seconds. Shape it into a round about 6-7 inches wide and 1 1/2-2 inches high, then place it on the prepared pan.

Use a sharp knife to cut a deep cross across the top, about 1/2-1 inch deep. Bake at 425°F (220°C) for 15 minutes, then reduce the oven to 400°F (200°C) and bake for another 25-35 minutes.

Bake until the loaf is deeply golden and sounds hollow when tapped on the bottom, or until the internal temperature reaches approximately 190-200°F (88-93°C). Cool on a rack for at least 20-30 minutes before slicing.