Preheat oven to 375°. In a large bowl, stir together 3 cups flour, the sugar, baking powder, soda, and salt. Add butter and cut in with a pastry blender until pieces are pea-sized. Stir in caraway, if using, then raisins and currants. Add 1 cup buttermilk and stir until dough is evenly moistened; if any dry patches remain, sprinkle with a little more buttermilk and stir.
Gather dough into a ball and transfer to a lightly floured work surface. Knead dough just until smooth, 12 to 16 turns, and pat into a smooth ball. Re-roll work surface, then pat out dough into a flat 1 1/4-in.-thick round. Slash an X about 1/2 in. deep across full width of round. Drizzle dough with about 1 tsp. buttermilk and smear it with your hand to cover dough. Transfer dough to a rimmed baking sheet lined with parchment paper.
Bake until deep golden brown and a wooden skewer inserted in center of bread comes out clean, 30 to 35 minutes. Transfer to a rack to cool. Cut loaf in half, then cut into wedges or slices.
* Baking powders have different formulations; Rumford has a milder flavor and contains less sodium than some brands.
Make ahead: Up to 1 day, wrapped airtight at room temperature, or 1 month, frozen.