Cream the butter, sugar and lemon rind until light & fluffy. Work in the egg yolk, then stir in the flour and currants. Knead lightly until smooth. Leave the mixture in the fridge or a cool place for about 1 hour.
Roll out the dough on a lightly floured surface to 3mm thick. Cut into rounds with a fluted 7.5cm cookie cutter. Brush with a little egg white, dredge with Caster Sugar, lift onto a baking paper lined tray & bake for 12–15 minutes or until just golden brown.
Lift onto a wire rack & leave to cool. Store in an airtight container.