Preheat oven to 180C/350F/gas 4.
Grease the baking sheets with butter.
Place the butter and sugar in the large mixing bowl and beat with the electric hand whisk until light and fluffy.
Add the eggs, one at a time, beating well after each addition.
Add the Cassia oil and beat until mixed in.
Sift the flour and spices into the mixture and fold in thoroughly. This is quite difficult with a spoon as the mixture is very stiff. I tend to use my hands to massage everything together.
Stir in the currants and lemon zest. Again, you may find it easier to massage these in.
Shape into a ball, wrap in clingfilm and place in the fridge for at least two hours.
Remove from the fridge, unwrap and place on a lightly floured work surface.
Roll the dough out to about 4mm thick.
Cut out the biscuits with the pastry cutter, re-rolling leftover dough to cut out more.
Place on the prepared baking sheets and place in the fridge for 15 minutes.
Sprinkle the biscuits with extra sugar and bake for 10-15 minutes. Don't let them brown round the edges. The biscuits should be pale in colour.
Cool on the baking sheets for 5 minutes before transferring to the cooling rack.
Allow to come to room temperature before serving.
