In a bowl, combine the dry ingredients (flour, bicarbonate soda, cornflour and salt) then set aside. In a small bowl, whisk together the egg and vanilla extract and also set aside.
In a stand mixer bowl, cream together the butter and sugar using the paddle attachment on a medium setting. You can also do this with a hand mixer. Once the butter is light and fluffy, add the vanilla extract and egg then continue to mix. Add in the dry ingredients and mix until well combined then mix in the chocolate chips. I normally keep a few chocolate chips leftover to add to the top of the cookies later.
Take a small amount of dough (around 30g) and roll it into a ball then place it on a large plate. Repeat this step until all the cookie dough is all used up. Place the dough balls in the fridge for around 2 - 3 hours or transfer them to a freezer bag and place them in the freezer for around 1 hour.
Preheat the oven to gas mark 4/350oF/180oC/160oC fan and line a baking tray with parchment paper. Place the cookie balls on the tray ensuring there is some space between each one as they will spread a little. I normally bake the cookies in batches to avoid cramming them on the tray.
Insert the leftover chocolate chips into the tops of the balls then bake the cookies for 11 minutes. Remove them from the oven and allow them to sit on the baking tray for 10 minutes to become firmer then transfer the cookies to a cooling rack to completely cool.