In a large bowl, use a hand-held mixer to cream the softened butter with both sugars until light and fluffy.
Add room temperature eggs, 1 at a time, and mix until just combined. Don't over mix.
Add the egg yolk and vanilla and mix until combined. Scrape down the sides of the bowl with a rubber spatula as needed.
In a medium bowl, whisk the flour, baking soda, baking powder, cornstarch, and salt.
Add the dry ingredients to the wet ingredients in 2-3 additions and mix until just incorporated after each. Don't overmix. Scrape down the sides of the bowl as needed. Once all of the dry ingredients have been added mix until smooth. Fold in the chocolate chips.
Cover the dough and refrigerate for at least 1 hour and up to 2 days.
Adjust your oven rack to the middle position and preheat your oven to 325°F and line 2 large baking pans with ungreased parchment paper or silicone baking mats.
Form the dough into balls that are about 1 and ½ tbsp. big. Arrange the dough on the prepared baking pans 1-2 inches apart.
Bake in the preheated oven, one pan at a time, for 15 minutes. The cookies will be very soft and will continue to cook and set as they cool on the pan.
Let the cookies cool on the pan for about 5 minutes before using a spatula to transfer them to a plate or a wire rack.
Cookies taste best warm and fresh out of the oven. Cover leftovers and store at room temperature for 1 week.