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

Melt the butter in the microwave.

In a medium bowl, whisk the melted butter, brown sugar, and white sugar together until fully incorporated.

Whisk in the egg and then the egg yolk into the butter mixture.

Finish the wet ingredients by whisking in the vanilla.

Pour the wet ingredients into the dry ingredients and mix together with a rubber spatula. The dough will be very soft, yet thick.

Next, fold in the chocolate chunks.

Cover the dough and chill for at least 2 hours - longer is better - overnight is great . Chilling is the key to making this recipe work.

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 the baking sheet with parchment paper or silicone baking mats. Set aside.

Using a small ice cream scoop, scoop out the dough and roll into a ball. (You want the dough to be taller than wide).

Bake the cookies for 11-12 minutes.

Allow to cool.

Serve and enjoy!