Beat together the butter and the sugar until fluffy.
Stir in the flour and currants until it combines to form a soft dough. If the dough needs a bit of help to come together add a splash of milk.
Roll out the dough on a lightly floured surface until around 5mm thick. Cut out shapes and place on a lined baking tray. Brush the biscuits with beaten egg then dust with the remaining caster sugar.
Bake at 200°c for 10 minutes until biscuits are golden. Cool on a wire rack then store in an airtight container.