Preheat oven to 350 degrees F. Spray a 8.5 x 4.5-inch loaf pan with cooking spray and set aside.
In the bowl of a stand mixer, combine flour, sugar, baking powder, baking soda and salt. Add the cold butter and mix on low until the butter is incorporated into the flour. The little pieces of butter should resemble small peas throughout.
In a separate bowl, whisk together egg, buttermilk, sour cream and orange zest. Add the egg mixture to the dry ingredients. Mix on low speed just until combined and no dry pockets of flour remain. Stir in the flour-coated currants and caraway seeds (if using).
Transfer to the prepared loaf pan. The dough will be very sticky, so it helps to dampen your fingers or spatula with a little bit of water so that you can spread the dough in the pan without too much sticking. Bake for 35-45 minutes, or until a toothpick inserted in the center comes out clean.
Remove from pan and cool completely on a wire rack before slicing.