Preheat oven to 375° F (190° C). If using a cast-iron skillet or Dutch oven, place inside of oven. If using a baking sheet, set aside.
In the bowl of an electric mixer, combiner flour, sugar, baking soda, and salt. Use the paddle attachment to mix, then add the chilled cubes of butter until butter is incorporated into the flour.
In a large measuring cup, lightly beat starter, buttermilk, egg, and orange zest.
With the mixer on low speed, add the starter mixture into the flour mixture.
Flour a work surface or cutting board. Toss the currants in the flour on the board, then add the currants into the dough.
Dump the dough onto the floured board and knead to form a round loaf.
Take a sheet of parchment paper and wet it. Wring out the excess moisture and place the dough ball on top. Lightly slice an "X" into the top of the dough with a serrated knife.
Hold the ends of the parchment paper to transfer the dough to baking sheet. If using a preheated cast iron skillet or Dutch oven, carefully add the dough and parchment paper into the center. Careful, it will be hot!
Bake for 50-60 minutes, or until a wooden skewer inserted into the center of the bread comes out clean. When you tap on the loaf, it should sound hollow.
Allow to fully cool on baking rack before slicing. Enjoy!
The damp parchment paper will create a little steam while baking, resulting in a crisper crust.
To avoid over-browning the bottom of your bread, insert an empty baking sheet on the rack below the bread.
If your bread begins to brown too quickly while baking, loosely tent some foil above it, shiny side up. Don't remove your bread from the oven, just open the oven door and drop the foil on top.
Allow the bread to fully cool before slicing, at least one hour (some folks swear by two). This is important, otherwise your bread may end up gummy on the inside if you cut into it too early.