Sift the flour and spices into a bowl. In a separate bowl, beat butter and sugar with electric beaters until thick and pale, then add eggs, 1 at a time, beating well after each addition. Fold in the flour mixture in 2 batches, then stir in currants and zest. Shape into a disc, enclose in plastic wrap and chill for at least 2 hours.
Line 2 baking trays with baking paper.
Divide dough into 4 portions. Roll out 1 portion to 4-5mm thick (keep remaining dough covered and chilled), then cut with a fluted 6cm round biscuit cutter and place on the trays. Repeat with remaining dough, re-rolling the trimmings to make about 40 biscuits. Chill for a further 15 minutes.
Preheat the oven to 180°C.
Sprinkle biscuits with extra sugar. Bake for 10 minutes until firm to touch but still pale. (Rotate trays halfway through baking, if necessary, to ensure even cooking.) Cool on trays for 5 minutes, then transfer to a rack to cool completely. The biscuits will keep in an airtight container for 3 days.
