Cream the butter and sugar together until pale and fluffy, then beat in the egg yolk. Sift in the flour and spices and mix well. Add the fruit and peel and enough brandy or milk to give a fairly soft dough. Knead lightly on a lightly floured surface and roll out until about 0.5 cm ( 1/4 inch) thick, prick the top with a fork and cut into 6 cm (2 1/2 inch) rounds using a fluted cutter.
Put onto greased baking sheets, bake for 10 minutes. Remove from the oven, brush with the lightly beaten egg white, sprinkle with caster sugar and return to the oven to bake for about 5 minutes longer, until the tops are golden brown. Transfer to wire racks to cool. Store in an airtight container.