1/4 cup unsalted butter, cut into small pieces and chilled
1 1/2 cups buttermilk
Position a rack in the middle of the oven and preheat to 400 degrees F. Lightly butter a baking sheet.
Put the currants in a medium bowl and cover with hot water. Soak for 15 minutes and drain.
In a large bowl, whisk together the flour, cake flour, sugar, baking soda, and salt. Add the butter and, using your hands, work it into flour mixture, until it is about the size of small peas. Stir in the currants.
Add the buttermilk and stir with a spatula, just until the flour is moistened and comes together in a shaggy dough. Turn the dough out onto a floured work surface and knead lightly until just smooth, about 1 minute. Form the dough into a ball and place on the center of the prepared baking sheet.
Using a sharp knife cut a 1/4-inch-deep "X" into the top of the dough to let the fairies out. Bake until well browned and the bottom sounds hollow when tapped lightly with your knuckles, about 1 hour.
Transfer the bread to a rack and let cool completely before serving.