Preheat your oven to 180°c (160°c for fan assisted oven) or Gas Mark 3 and grease a couple of baking trays with a little butter.
In a large bowl, cream together the butter and caster sugar until it becomes light and fluffy.
Sift in the flour and cornflour and mix to form a stiff dough, using your hands if necessary. Tip the dough onto a lightly floured worktop and gently knead until smooth.
Roll your dough into a ball before forming it into a long sausage (about 5cm in diameter). Wrap the sausage-shaped dough in some cling film and transfer to the fridge. Leave for about 30 minutes to stiffen slightly.
Place the demerara sugar into a spare baking tray, before removing the dough from the fridge and roll the sausage-shaped dough through the sugar, ensuring it's completely coated.
Cut the sugar-coated dough into circles around 1cm thick and transfer to your pre-greased baking trays. You should get about 18 biscuits.
Bake in your pre-heated oven for around 10-15 minutes until the tops are lightly golden.
Remove from the oven and leave the biscuits to cool on their tray for around 10 minutes, before transferring to a wire rack to cool completely.
Granny likes to add a little cinnamon to her demerara sugar.