In a large bowl, combine flour, baking soda, cornstarch and salt.
In a separate bowl, whisk together the melted butter and sugars until no brown sugar lumps remain.
Whisk in the egg, then the egg yolk. When combined, add the vanilla.
Pour the wet ingredients into the dry ingredients and mix together with a large spoon or rubber spatula. This dough will be soft.
Fold in chocolate chips.
Cover dough and refrigerate for a minimum of 2 hours. Overnight is best.
Remove dough from refrigerator and soften at room temperature, for about 10 minutes.
Preheat oven to 325°.
Place rounded balls, about the size of golf balls, on parchment lined baking sheets. Rolling the balls between your hands will help them hold together.
Bake cookies for 11-12 minutes, and no longer! Overbaking will ruin the cookies. Allow cookies to cool on baking sheet for 10 minutes, before moving to a wire rack.