Makes about 16 tea scones.
Preheat the oven to 375ºF (190ºC) and line baking trays with parchment paper.
Sift the flour, sugar, baking powder and salt into a large mixing bowl. Add the grated orange zest ad mix in. Add the butter and cut this in using a pastry cutter, two butter knives or your fingertips, until no large pieces are visible and the mixture is a rough, crumbly texture. Use your hands to rub the mixture between your palms to flatten any butter pieces – this helps to make the scones flaky.
In a separate bowl, whisk the milk, egg and egg yolk together. Make a well in the centre of the flour mixture and add the milk all at once. Use a spatula or wooden spoon to mix everything together just until it starts to come together but it is still rough and crumbly. Turn everything out onto a clean surface and sprinkle the currants overtop. Use your hands to bring the dough together, and to flatten and fold the dough over a few times – this works the currants into the dough and helps build layers to promote flakiness.
Flatten the dough with your hands, to just under an inch (2.5 cm) thick and use a 2 ½-inch (7.5 cm) round cutter to cut out scones. Arrange these on the baking tray, leaving 2 inches (5 cm) between them. Brush the tops of the scones with the egg white and sprinkle with cinnamon sugar. Bake the scones for about 18 minutes, until golden brown on the bottom and lightly browned on top. Cool the scones on a rack.
The scones are best enjoyed the day they are baked.