Preheat the oven to 170C (fan)/Gas 5
Line three or four baking trays with baking paper.
Place the butter and sugar in a bowl and beat until well pale and creamy.
Add the egg yolks, beating well.
Sift in the flour and spice mixing well.
Stir in the currants and peel and knead the dough very lightly to form a ball. ( add a little of the leftover egg white if too stiff)
Turn out on to a floured work surface and, using a rolling pin, roll out the dough until 5mm/¼in thick.
Using a 8cm/3in cutter, cut out 24 rounds.
Transfer to the baking trays and bake for approx 15 minutes until pale golden brown and cooked though. Sprinkle with caster sugar as soon as they are baked.
Leave to cool on the trays for a few minutes, then transfer a wire rack to cool completely.