01 - Preheat oven to 400°F. Line baking sheet with parchment paper.
02 - Whisk together all-purpose flour, whole wheat flour, sugar, baking soda, baking powder, and salt in a large bowl.
03 - Add cold butter cubes. Rub into flour mixture using fingertips or pastry cutter until mixture resembles coarse crumbs.
04 - Stir in currants until evenly distributed throughout the mixture.
05 - Whisk together buttermilk and egg in a separate bowl.
06 - Pour wet ingredients into dry mixture. Stir gently just until combined to form a shaggy dough. Do not overmix.
07 - Turn dough onto lightly floured surface. Pat into a 1-inch thick round. Cut into 8 wedges and transfer to prepared baking sheet, spacing slightly apart.
08 - Brush tops with buttermilk and sprinkle with coarse sugar if desired.
09 - Bake for 18 to 20 minutes until golden brown and a toothpick inserted in the center comes out clean.
10 - Cool on wire rack. Serve warm or at room temperature.