Preheat the oven to 175°C (350°F).
Line and spray a 20cm (8 inch) springform tin.
In a large bowl for your stand mixer cream together the butter, brown sugar and caster sugar for 3-4 minutes. You should scrape the side of the bowl often. You want a light and creamy texture. (see note)
Add the egg, egg yolk and vanilla and mix for 1 minute.
In a separate bowl combine the flour, cornflour and baking powder and give it a whisk together.
Add flour mix to your stand mixer and mix for 1 minute on low until just combined.
Remove bowl from your stand mixer and stir through your add ins like chocolate chips.
Spread cookie dough in to the prepared can tin.
Bake for 25 minutes but start checking at 20 minutes with a skewer. Different ovens will affect the total cooking time. Mine took 25 minutes for the skewer to come out clean. (see notes)
Set aside to cool for 10 minutes then run a flat knife around the outside to loosen the cookie. Leave it in the tin to cool completely.
Remove from tin and place on serving plate ready to decorate.