Preheat your oven to 375°F (190°C) and grease a 10-inch cast-iron skillet with 1/2 tablespoon of butter.
In a large mixing bowl, whisk together flour, sugar, sea salt, and baking soda.
Add the cold butter chunks to the dry mixture and work it into the flour using your fingers or a pastry cutter until the mixture resembles coarse crumbs.
If using, stir in the raisins or dried cranberries.
Form a well in the center of the dry ingredients. In a separate measuring cup, whisk together the cold buttermilk and egg, then pour into the well.
Mix gently until just combined; do not overmix.
Transfer the dough to a floured surface and shape it into a round loaf.
Place the loaf into the greased skillet and score the top with a deep 'X' using a sharp knife.
Bake for 50-55 minutes, or until the bread sounds hollow when tapped on the bottom.
Transfer the bread to a wire rack to cool before slicing.
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Notes
Store wrapped in a clean cloth or in an airtight container at room temperature for a few days.