Mix together the melted butter, brown sugar, and white sugar until no lumps remain.
Add in the egg, then the egg yolk.
Add the vanilla.
Slowly mix in the flour, baking soda, cornstarch, and salt, until combined. The dough will be very soft, yet thick.
Fold in the chocolate chunks, evenly distributing throughout the dough.
Cover and chill for at least 2 hours, up to 3 days. With this particular dough, you really can’t skip this step.
When ready to bake, take the dough out of the refrigerator and allow to slightly soften at room temperature for 5 minutes.
Preheat the oven to 325 degrees. Line two large baking sheets with parchment paper or foil (lightly grease, if using foil).
Roll the dough into 1.5 – 2″ balls. The dough will be crumbly, but the warmth of your hands will hold it together. Roll your dough balls to be taller than wide, to help prevent them from spreading too thin.
Space the dough balls about 2″ apart. Bake for 10-12 minutes.
The cookies will look very soft and under baked. Allow to cool on the cookie sheet for 10 minutes before moving to a wire rack to cool completely.