Preheat oven to 375 degrees. Line a sheet pan with parchment paper. Combine flour, sugar, baking soda, and salt in the bowl of an electric mixer fitted with a paddle attachment. Add butter and mix on low speed until butter is mixed into the flour.
With a fork, lightly beat the buttermilk, egg, and orange zest together in a measuring cup. With mixer on low speed, slowly add buttermilk mixture to flour mixture.
In a small bowl, mix currants with 1 tablespoon of flour, then mix into the dough. Dough will be very wet.
Turn the dough out onto a well-floured board and knead a few times into a round loaf. Place the loaf on prepared sheet pan and lightly cut an X into the top of the bread with serrated knife.
Bake for 45- 55 minutes, or until a toothpick or wooden skewer comes out clean. When you tap the loaf, it will have a hollow "thump" sound. Cool on a baking rack. Serve warm or at room temperature.
Sugar: For a less-sweet Irish soda bread, reduce sugar to 2 or 3 tablespoons.