Combine butter and sugar in a bowl and cream with electric beaters for 5 minutes or until pale and fluffy.
Add eggs, one at a time, beating well between each addition.
Sift flour, salt and spice mix and fold into the butter mixture.
Fold in the currants and orange zest and turn mixture out onto a lightly floured work surface. Form into a disc-shaped dough, cover with plastic and place in the fridge to chill for at least 1 hour.
Preheat oven to 180C and line 2 baking trays with paper.
Roll out dough between 2 sheets of baking paper until about 5mm thick.
Cut out rounds with a fluted cookie cutter and place with a little space between each biscuit onto the baking trays.
Sprinkle with the extra caster sugar and bake for 15-20 minutes or until lightly golden.