Line a baking tray with parchment and pre-heat the oven to 350F.

Cream the butter, sugar and lemon rind until light & fluffy. Work in the egg yolk, then stir in the flour and currants. Add a little milk if needed. Knead lightly until smooth. Flatten and wrap in plastic wrap and place in the fridge for an hour.

Roll out the dough on a lightly floured surface to 1/8 of an inch thick. Cut into rounds with a 3 inch round cookie cutter, fluted in you have. 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.