Put the flour, salt and butter into a mixing bowl and rub to the fine crumb stage.Then add the sugar and after that enough milk to mix to a firm dough that will leave the bowl clean. After that transfer it to a lightly floured surface and roll it out to a rectangle 20cm by 30cm. Now sprinkle the currants over half the surface and then fold the other half on top and roll everything again so you end up with a rectangle 20cm by 30cm. Then trim it neatly using a sharp long-bladed knife, so you end up with a shape about 18cm by 28cm.Cut this into 24 fingers approximately 3cm by 7cm. Now place the biscuits on the baking sheet, brush with a little egg white and sprinkle with granulated sugar. Bake near the centre of the oven for 12–15 minutes, then cool on a wire tray and store in an airtight tin.