Whisk together the flour, baking soda, cornstarch and salt in a large bowl. Set aside.

In a medium bow, whisk together the melted butter, sugars until no lumps remain in the brown sugar. Whisk in the egg, egg yolk and vanilla. Pour the wet ingredients into the dry ingredients and mix together with a rubber spatula or wooden spoon. The dough will be soft and thick. Fold in the chocolate chips. Cover the dough and place in the refrigerator up at least 3 hours, or up to 3 days. Chilling is mandatory. Do not skip this step.

Take the dough out of the refrigerator and allow to slightly soften at room temperature for about 10 minutes.

Preheat the oven to 350F degrees. Line two baking sheets with parchment paper or silicone baking mats (preferred.) Set aside.

Roll the dough into balls, about 2 Tablespoons of dough for each. Roll the cookie dough into tall balls (will help the cookies be thick.) Place 8 total cookie dough balls on each sheet. I like to press a few more chocolate chips into the top of each. Bake for 11-12 minutes. The cookies will look underbaked, but that's what you want. They will continue to cook on the baking sheet. Let the cookies sit on the sheet for 10 or so minutes before transferring to a cooling rack.

Store in an airtight container for up to 7 days.