Mix the butter and sugar together (preferably with a wooden spoon) until it is pale and creamy.
Sieve both the flour and the cornflour into the bowl and mix well.
Put a small amount of flour on your working surface and place the dough on this.
Shake a little flour on top and roll out about quarter inch thick.
Prick with a fork and cut into rounds with a cutter
or, if you want one large shortbread round, pinch the edges with thumb and finger all round.
Use a palette knife to lift the shortbread onto an oiled baking tray and bake for 25 minutes in a pre-heated oven at 325F/170C/Gas Mark 3.
If the biscuits are ready, they will be pale brown and crisp; if not, return to the oven for a further 5 or 10 minutes.
Shake a small amount of caster/granulated sugar on the top of the shortbread immediately after they have been removed from the oven.
Use a palette knife to move them to a wire cooling rack
Store in an airtight tin once they are cold