In a large bowl, whisk flour, 3 tbsp (45 mL) of the sugar, the baking powder, baking soda and salt. Using a pastry blender or fingertips, mix in butter until it forms coarse crumbs. Stir in currants and lemon zest.
In a small bowl, whisk ½ cup (125 mL) of the buttermilk with egg; add to flour mixture, stirring just until incorporated.
Turn dough out onto lightly floured surface; pat into ½-inch (1 cm) thickness. Using a 1½-inch (3.5 cm) round cutter, cut out scones. Press any scraps together and repeat with remaining dough to make about 24 scones.
Place 1 inch (2.5 cm) apart on parchment-paper-lined baking sheet. Brush tops with remaining buttermilk; sprinkle with remaining sugar. Bake in a preheated 400°C (200°C) oven until light golden on the outside and fluffy inside, 12 to 14 minutes.
Per Scone: 73 calories, 1 g protein, 3 g fat (2 g saturated fat), 10 g carbohydrates, 0 g fibre, 17 mg cholesterol, 59 mg sodium