Preheat the oven to 200°C/400°F/Gas mark 6. Lightly grease two baking trays.
Cream together the butter and sugar, and beat in the egg yolk.
Sift together the flour, salt and spices. Fold into the creamed mixture with the currants and peel. Add the milk to form a soft dough.
Knead lightly on a floured work surface, and roll out to 8mm/1⁄4in thick. Cut out using a 6cm/2 1⁄2in fluted cutter, and put on the baking trays.
Bake in the oven for 10 minutes, then brush with egg white and sprinkle with caster sugar. Return to the oven for a further 10–15 minutes.
Remove from the oven, and carefully slide onto a wire rack to cool.