Add the room temperature butter, granulated sugar, brown sugar, salt and vanilla to a large bowl and cream with an electric hand mixer until creamy and light in colour (about 2-3 minutes).

Add egg and egg yolk, and mix until combined and creamy.

Add the flour, cornstarch, baking powder, and baking soda and mix until just combined.

Add chocolate chunks, and fold with a spatula until the mixture is just combined. Don't over-mix.

Cover the bowl and let the dough rest in the fridge for 2 hours.

Preheat oven to 350°F.

Line 2 baking sheets with parchment paper.

Scoop 100g of cookie dough (weigh to be precise) and shape into a ball.

Bake 3 cookies at a time, leaving space between each ball, and chill the cookie dough balls in the fridge while you wait for each batch to bake.

Bake for 12-15 minutes or until the cookies spread and the top is light golden brown. Do not overbake as they must be soft in the middle.

Remove from the oven and top with more chocolate chunks and give the baking sheets a good tap to spread the cookies out a bit.

Let the cookies cool for 5 minutes, and then transfer to a wire rack to cool completely.