Preheat the oven 200C/180C Fan/Gas 6. Line 3 baking trays with non-stick paper.
Measure the butter and sugar into a bowl and cream until well combined and fluffy. Add the egg yolk and lemon zest. Sift in the flour and mix well to combine. Stir in the currants and enough milk to make a fairly soft dough.
Knead the dough on a floured work surface and, using a rolling pin, roll the dough until about 5mm/Â¼in thick. Using a 6cm/2Â½in round fluted cutter, cut out 24 rounds. Transfer to the baking trays using a palette knife.
Bake the biscuits for about 8 minutes. Meanwhile, lightly beat the egg white using a fork.
Remove the biscuits from the oven and brush the tops with beaten egg white. Sprinkle over the caster sugar and return to the oven for further 5 minutes, or until pale golden-brown and cooked though. Leave to cool on the trays for a few minutes, then carefully lift on to a wire rack to cool completely.