Makes 16 large cookies. Cookies stay soft and fresh for 7 whole days at room temperature. Cookies may be frozen up to 3 months. Rolled cookie dough may be frozen up to three months and baked in their frozen state for 12 minutes.
Toss together the flour, baking soda, cornstarch and salt in a large bowl. Set aside.
In a medium size bowl, whisk the melted butter, brown sugar, and white sugar together until no brown sugar lumps remain. Whisk in the egg, then the egg yolk. Finally, whisk in the vanilla. Pour the wet ingredients into the dry ingredients and mix together with a large spoon or rubber spatula. The dough will be very soft. Fold in the chocolate chips. They may not stick to the dough because of the melted butter, but do your best to have them evenly dispersed among the dough. Cover the dough and chill for 2 hours, or up to 3 days. Chilling is mandatory.
Take the dough out of the refrigerator and allow to slightly soften at room temperature for 10 minutes.
Preheat the oven to 325F degrees. Line two large baking sheets with parchment paper or silicone baking mats. Set aside.
Roll the dough into balls, about 3 Tablespoons of dough each. The dough will be crumbly, but the warmth of your hands will allow the balls to stay intact. Roll the cookie dough balls to be taller rather than wide, to ensure the cookies will bake up to be thick. See this post for more clarity and a photo. Put 8 balls of dough onto each cookie sheet. Press a few more chocolate chips/chunks on top of the dough balls for looks, if desired. Bake the cookies for 11-12 minutes. The cookies will look very soft and underbaked. They will continue to bake on the cookie sheet. Allow to cool on the cookie sheet for 10 minutes before moving to a wire rack to cool completely.
****Room temperature egg + egg yolk are preferred for even disbursement. Typically, if a recipe calls for room temperature or melted butter, it's a good idea to use room temperature eggs as well. To bring eggs to room temperature quickly, simply place the whole eggs into a glass of warm water for 10 minutes.