Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper and set them aside.
In a stand mixer, beat the softened butter with the brown and granulated sugars for about 3 minutes until light and fluffy. Mix in the egg and vanilla until well combined.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gently sift these dry ingredients into your butter mixture. Mix on low speed until just combined – don't overmix!
Use a spatula to fold the chocolate chips into the brownie dough. Set this delightful dough aside.
Now, in your stand mixer, cream together the cold, cubed butter with both brown and granulated sugars for about 4 minutes until the mix is pale and super fluffy, with no butter chunks. Scrape the bowl.
Beat in the egg and then the egg yolk, one at a time, scraping the sides of the bowl after each addition. Stir in the vanilla until blended.
In a new bowl, whisk together the flour, cornstarch, baking soda, and salt. Gradually add these dry ingredients to your butter and egg mixture in two parts, mixing on low speed until just combined.
Gently fold in the chocolate chips with a spatula. Avoid overmixing to keep your cookies tender.
Scoop equal portions of both brownie and chocolate chip doughs. For the best look, roll all your brownie dough into balls first, then all your chocolate chip dough.
Take one brownie dough ball and one chocolate chip dough ball. Press them together side-by-side in your hands, then gently roll them into one combined ball. Place these magical marbled balls on your prepared baking sheets, leaving about 2 inches between each.
Bake for 8 to 9 minutes. The edges should be set, but the centers will still look puffy and slightly soft – they'll firm up as they cool. Do not overbake!
Let the cookies cool on the baking sheet for 5-10 minutes before moving them to a wire rack to cool completely. For an extra flourish, sprinkle with flakey sea salt before serving, if you like!