Preheat  the oven to 300°F. Butter a 9in x 13in Swiss roll baking tin. Place the butter  and caster sugar in a food processor  and blend until pale and fluffy.

Add the sifted flour and cornflour and blend the mixture briefly again,  just until thoroughly combined. Tip the mixture into the prepared baking tin and,  using floured hands,  press down  so it is level all over.

Pierce all over with a fork (do this carefully, so that you do not disturb the level surface) and then  bake for 50-60  minutes.  What you are looking for is a uniform pale golden  colour all over. Do not allow it to become  golden  brown.

Remove the shortbread from the oven and dredge with sugar,  then  cut into squares.  Leave for 5 minutes  or so, then  carefully transfer  to a wire rack to cool completely.

To serve, arrange  the shortbread on a large plate and place on the table so that  guests  can help themselves.