01 - Preheat oven to 400°F. Line a 12-cup muffin tin with paper liners or lightly grease with butter.
02 - In a large bowl, whisk together all-purpose flour, whole wheat flour, sugar, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, whisk together buttermilk, egg, and melted butter until thoroughly combined.
04 - Pour the wet ingredients into the dry ingredients. Add currants and orange zest if using. Gently fold together until just combined; do not overmix.
05 - Divide the batter evenly among the muffin cups, filling each about 3/4 full.
06 - Bake for 18–20 minutes, or until golden and a toothpick inserted into the center comes out clean.
07 - Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.