Warm the mixing bowl of a stand mixer, not too hot though.
Add butter and sugar to the bowl and beat until creamed, which should take between one and two minutes depending on how soft your butter is and your mixer speed; number 4 to number 6 speed.
Sift flour and cornflour into a separate bowl and then add half to your creamed butter and sugar.
Start your mixer on slow (number 2 speed) to prevent the flour flying out of the bowl – mix for a few seconds. Add the remainder of the flour mix and combine until it all comes together around the beater – about 1 minute.
Alternatively, you can 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 the dough on a floured work surface to 30 cm x 23 cm (12 ins by 9 ins) and place in your tin and prick all over with a fork.
Alternatively, press the shortbread dough with your fingers easing into a rectangle shape in your tin and prick all over with a fork.
Place in the oven and bake for 20 minutes.
Leave to cool for 15 minutes.