Preheat your oven to 350°F and line two large baking sheets with parchment paper. Set them aside.
To prepare the brownie cookie dough, cream together the butter and brown sugar in the bowl of a stand mixer for about 3 minutes, until the mixture is light and fluffy. Add the egg and vanilla, mixing until well combined.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Sift the dry ingredients into the butter and egg mixture. Mix on low speed until fully incorporated and the flour is blended in.
Stir in the chocolate chips using a large spatula. Set the brownie dough aside while you prepare the chocolate chip cookie dough.
For the chocolate chip cookie dough, place the cold butter, brown sugar, and granulated sugar in the bowl of a stand mixer fitted with the paddle attachment. Begin mixing on low speed, gradually increasing the speed to cream the butter and sugars together for about 4 minutes, until the mixture is pale and fluffy. Scrape down the sides of the bowl as needed, ensuring no butter chunks remain.
Add the egg and egg yolk one at a time, scraping down the sides of the bowl between each addition. Then, mix in the vanilla extract.
In a separate bowl, whisk together the flour, baking soda, cornstarch, and salt.
Add the dry ingredients to the butter mixture in two additions, mixing on low speed until just combined. Be careful not to over mix.
Use a large spatula to fold in the chocolate chips, again being careful not to over mix.
Next, scoop the doughs into balls using a small cookie scoop. Make the chocolate chip cookie dough balls slightly larger than the brownie cookie dough balls. Roll them into uniform balls using the palms of your hands. For consistent-sized cookies, use a kitchen scale to weigh the dough balls. It’s easiest to roll all of the brownie dough balls first and then roll all of the chocolate chip dough balls.
Take one brownie cookie dough ball and one chocolate chip cookie dough ball, press them together, and roll them into one cohesive ball using your hands. Place the combined dough balls on the prepared cookie sheet, spacing them about 2 inches apart.
Bake one sheet at a time in the center of the oven for 8 to 9 minutes, or until the edges are set and the tops are slightly underbaked and fluffy. Be careful not to overbake.
Remove the cookies from the oven and let them cool on the cookie sheet for 5 to 10 minutes before transferring to a wire rack to cool completely. Repeat with the remaining dough, ensuring the cookie sheet is cooled between batches.
Optionally, sprinkle the cookies with flaky sea salt before serving.
Note: Depending on how you divide the chocolate chip dough and brownie dough, you may have some chocolate chip cookie dough left over. If so, bake the remaining cookies at the end.