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.