Lightly grease two baking trays.
Cream the butter or margarine with the sugar until very soft, light and fluffy.
Beat in the egg yolk.
Sift together the flour and spice, then gradually work these dry ingredients into the creamed mixture, together with the peel and currants and enough milk to make a firm dough.
Knead the dough lightly and roll it out on a floured board to a thickness of 3mm/1/8 in.
Cut the biscuit dough into rounds using a 7.5 cm/3 in fluted cutter.
Carefully transfer the biscuits to the prepared baking trays and prick each one three or four times with a fork.
Bake in a moderate oven (180C, 350F, gas 4) for about 20 minutes, or until lightly browned.
Immediately they are removed from the oven, sprinkle the biscuits with caster sugar.
Leave the Easter biscuits to cool on the trays for a minute before transferring them to a wire rack to cool completely.