In a large mixing bowl, whisk together flour, baking soda, cornstarch and salt.
In a separate bowl, mix brown sugar and cane sugar into the melted butter until the sugars have dissolved. Mix in the egg and egg yolk until combined. Stir in the vanilla extract.
Slowly stir in the flour mixture.
Fold in the chocolate chips.
Cover and chill in the fridge for at least 1 hour.
Preheat the oven to 325 degrees. Remove the cookie dough from fridge and let it sit for 10 minutes before rolling into balls about 2 tablespoons in size.
Bake for 8-10 minutes or until slightly golden on top. They will look underdone but this is what keeps them soft.
Let the cookies cool for 5-10 minutes before removing to cool completely on a wire rack.
