Add sugar and room temperature butter to the bowl of your stand mixer. Beat for a couple of minutes until creamy and smooth.
Add half the cornflour and plain flour and mix on low (number 2 speed) until it forms clumps. Then add the remaining flour and cornflour.
Mix for about 1 minute on a low speed until it starts to come together as a dough. Don't over beat or you will have tough shortbread.
With your hands, bring the dough mixture together and shape into a rough rectangle.
Wrap in plastic wrap and pop in the fridge for 15 minutes to firm the shortbread dough.
Remove the dough from the fridge and transfer to a floured work surface.
Roll out the dough to just less than 6 mm (¼”) thick for rounds or 12mm (½”) for fingers.
For rounds, cut the dough using a 6 cm (2 ½”) round fluted cookie cutter. For fingers cut to approximately 10 cm (4“) x 2.5 cm (1”)
Place on a baking tray, leaving space between each biscuit. For fingers, prick with a fork to allow for even baking.
Pop your baking tray of shortbread into the fridge for at least 30 minutes. Preheat your oven to 170℃/150°C fan/ 300°F/Gas Mark 3.
Remove your baking tray of biscuits from the fridge and bake in the preheated oven until pale golden in colour. This will take about 15 minutes for round shortbread biscuits or 25 to 30 minutes for shortbread fingers.
Leave biscuits to cool on the tray for 10 minutes before transferring to a wire rack to cool completely.
Optional: Dredge with caster sugar while shortbread is still hot.