Sprinkle some flour on a work top and roll out the ball to 1cm thickness. Cut out shapes with biscuit cutters.
Place shortbread on a baking tray and bake 20-25 minutes or until just golden brown. Remove from the oven and sprinkle caster sugar on top.
Combine all ingredients except the caster sugar in a bowl and rub with your fingers until it resembles breadcrumbs. At this point start squeezing the dough together, it should form a dough ball with no crumbs left in the bowl.