Add butter and sugar to the mixing bowl and beat until creamed. Depending upon the softness of your butter and your mixer speed, this should take between one and two minutes. I find a mixer speed of 4, 5 or 6 works well with my Kitchen Aid mixer.
Sift flour and cornflour into a separate bowl and then add half to your creamed butter and sugar.
Start with a slow speed on your mixer (to prevent flour flying everywhere) before mixing for a few seconds. Add the remainder of the sifted flours and combine until everything comes together around the beater – about a minute’s mixing.
Alternatively, cream the butter and sugar together with a hand held mixer or a wooden spoon in a large bowl. Add the flour and cornflour in 3 or 4 batches and mix until the dough comes together. Don’t overwork it.
Roll out your dough on a floured work surface to 30 cm x 23 cm (12 ins by 9 ins). Once placed in your tin, prick the dough all over using a fork.
Alternatively, press and shape the shortbread dough with your fingers, easing into a rectangle shape in your tin. Prick the dough all over with a fork.
Place in the oven and bake for 20 minutes before leaving to cool for 15 minutes.