Preheat oven to 410 degrees (not convection).
Prep two large aluminum cookie sheets with parchment paper and set aside.
In a stand mixer fitted with a paddle attachment, whip up the butter for 1 minute until it's light and fluffy, then add in the sugars and cream together for 3 minutes on high speed.
Scrape down the sides an add in the vanilla and eggs one at a time; whip up well between each addition.
Add in the flour, cocoa, cornstarch, baking soda, and salt (not the sea salt) and slowly mix together either by hand or on the very lowest speed. Do not overmix!
Reserve a little bit of the chips, then stir in the rest of them gently into the dough on the very lowest speed.Split the cookie dough in eight balls and place 4 cookie dough balls on each cookie sheet. Hand press on the reserved chocolate chips, and sprinkle on the sea salt. Bake for 9-10 minutes. They should be thick, tall, and gooey in the center. Let cool on the pan for 1 minute, then transfer to a cooling rack for 10 minutes to finish setting up.