Sift your flour and cornflour into bowl and pop this into a food processor, blitzing to combine. You can also do this in a bowl using your hands, but it’s handier in a food processor. Gradually add your butter while blitzing, it will start to come together at this point and resemble breadcrumbs. If it sticks to the sides or the bottom of the food processor, use a knife to scrap them down. Pop in your vanilla extract and blitz again to combine. Add your sugar and give it another wee blitz.