In a bowl, sift and whisk together the plain flour, bicarbonate soda, cornflour and salt then set aside. In a second bowl whisk together the egg and vanilla extract also set aside.
In a third bowl or stand mixer, cream together the butter, brown sugar and granulated sugar until creamy and fluffy and then pour in the egg and vanilla and continue to mix until well combined. Add in the dry ingredients and continue to mix to combine all the ingredients then fold in the chocolate chips.
Take a heaped tablespoon of dough and roll the dough into a ball then place the dough on a large plate lined with baking paper. Repeat this process until you use up all the dough. Wrap the plate with plastic wrap and place the plate in the refrigerator for 1 hour.
While the cookie dough is in the fridge, prepare a baking tray with baking paper and preheat the oven to gas mark 4/350oF/180oC/160oC fan. Remove the cookies from the fridge and arrange the cookies that you want to bake on the baking tray ensuring there is space for them to spread (around 10cm). If you do not have enough space on the tray then you can place the remaining cookie dough balls back in the refrigerator and bake them in batches or you can freeze the rest and bake as needed.
Bake the cookies in the centre of the oven for 12 minutes or until slightly brown. If you want, you can place a few chocolate chips on top of the cookies when you remove them from the oven. After 10 minutes, use a spatula and place the cookies on a cooling rack to completely cool then enjoy.