1. Preheat the oven to 350°F. Line two large baking sheets with parchment paper. Set aside.
2. To make the brownie cookie dough, mix together the butter and brown sugar in the bowl of a stand mixer for about 3 minutes, until light and fluffy.
3. Add the egg and vanilla, mixing to combine well.
4. In a separate bowl, whisk together the flour, cocoa powder, baking soda and salt. Sift the dry ingredients into the egg and butter mixture. Mix on low speed until combined and all of the flour is incorporated.
5. Use a large spatula to stir in the chocolate chips. Set the dough aside while you make the chocolate chip cookie dough.
6. To make the chocolate chip cookie dough, add the cold butter, brown sugar and granulated sugar to the bowl of a stand mixer fitted with the paddle attachment. Mix on low speed. Gradually increase the speed and cream the butter and sugars together for 4 minutes, until pale and fluffy. There should be no visible chunks of butter in the mixture. Scrape down the sides of the bowl.
7. Add the egg and egg yolk, one at time, stopping to scrape down the sides of the bowl between each addition.
8. Mix in the vanilla.
9. In a separate bowl, whisk together the flour, baking soda, cornstarch and salt.
10. Add the dry ingredients to the butter and egg mixture in two additions. Mix on low speed until just combined. Do not over mix.
11. Use a large spatula to stir in the chocolate chips. Do not over mix the dough.
12. Using a small cookie scoop, scoop the doughs into balls, making the chocolate chip cookie dough balls slightly larger than the brownie cookie dough balls. Use the palms of your hands to roll into uniform balls. For equal size cookies, use a kitchen scale to weigh the balls. It’s easiest to roll all of the brownie cookie dough balls and then roll all of the chocolate chip cookie dough balls.
13. Take a brownie cookie dough ball and a chocolate chip cookie dough ball, press them together and use your hands to roll the two pieces into one ball. Place on the prepared cookie sheet, about 2 inches apart.
14. 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 fluffy and slightly under baked. Do not over bake.
15. Remove the cookies from the oven and cool on the cookie sheet for 5 to 10 minutes before transferring to a wire rack to cool completely. Repeat with remaining cookie doughs, being sure to cool the cookie sheet between batches. 
16. Sprinkle with flakey sea salt before serving if desired.

**Note: Depending on the ratio of chocolate chip dough to brownie dough you use for the balls, you may have a bit of chocolate chip cookie dough left over. Bake any remaining cookies off at the end.