Remember these will need to chill overnight in the fridge!!
Whisk together flour (2 1/4 cups), cornstarch (1 teaspoon), baking powder (1/2 teaspoon), baking soda (1/4 teaspoon), and salt (1/2 teaspoon) in a medium bowl; set aside.
In the bowl of a stand-up mixer fitted with a paddle attachment, beat the butter (1 cup), brown sugar (3/4 cup), and granulated sugar (1/4 cup), for 2 minutes at medium-high speed, until mixture is fluffy.
Using a rubber spatula, scrape down the sides of the bowl, and add the eggs (1 egg + 1 egg yolk) and vanilla (2 teaspoons). Continue to beat on medium-high speed for 3 minutes.
Scrape down the sides, and gradually add the flour mixture. Mix on low speed until combined, just until almost combined, but Do No Overmix.
Remove bowl from the stand-up mixer and fold in 1 1/2 cups of chocolate chunks.
Line a baking sheet with parchment paper, and place a piece of wax paper over a food scale.
Use a large cookie scoop dough (over-filled, not leveled) or large ice-cream scoop to scoop the dough onto the food scale, and measure 4-ounce cookies, which should be around 1/2 cup of dough. Shape the dough so it's tall and round, and place it on your prepared cookie sheet. Press the remaining 1/2 cup of chocolate chunks into the top of your dough. Place cookies side by side, and into the fridge overnight to harden the butter completely.
Place the formed dough balls in the refrigerator on the cookie sheet overnight.
Preheat oven to 350°F. Place chilled cookies on each baking sheet, making sure to space them out as much as possible. Bake in preheated oven for 13-16 minutes (mine were perfectly baked at 15 minutes). You'll know the cookies are done once the edges are golden brown.
Enjoy. Sprinkle warm cookies with flaky sea salt, and allow them to cool on the baking sheet until they've firmed enough to transfer before moving them to a wire rack. Enjoy!