In a medium bowl, combine the dry ingredients (flour, cocoa powder, cornstarch, baking soda, and salt). Set aside
In the bowl of a stand mixer fitted with a paddle attachment, or a large bowl with a hand mixer, add the room temperature butter, the brown sugar, and the granulated sugar. Beat on medium-high speed until light and fluffy, 3–5 minutes. Scrape down the sides of the bowl and the paddle, then add in the egg and vanilla. Mix again on medium speed to combine.
Turn the mixer off and add in half of the dry ingredients. Turn the mixer to low speed to combine, then add in the other half of the dry ingredients. This is just to avoid the dry ingredients spilling everywhere. Add in the milk and mix again until a smooth but sticky dough forms. Add in the chopped chocolate and mix until combined. Cover the dough bowl with plastic wrap and refrigerate for at least 3 hours. The longer the better.
Preheat oven to 350°F. Scoop giant 5-ounce balls of dough and place onto a cookie sheet lined with parchment paper. Bake for 15–17 minutes, until edges are set and cookies are thin but have shiny centers that look only slightly underdone. Halfway through the cookies baking, open the oven, remove the pan, and tap it against the counter to deflate the cookies, then place them back in the oven. When cookies are done baking, top with flaky salt and extra chocolate chunks!Note: If you scoop cookies in a different size, the bake time will change.
