Heat the oven to 350°F.
Very lightly oil the baking pan and line it with parchment paper. The oil helps keep the parchment from moving around on you, and the parchment makes it easy to remove the brookies without sticking.
To make the brownie batter, melt the butter and whisk in the cocoa powder. Whisk in the sugar and salt, followed by the eggs and vanilla. Whisk the eggs and sugar for a couple of minutes to ensure they’re combined very, very well.
Add in the flour and mix until nearly combined, then fold in the chocolate chips or other additions (like nuts, etc.). Make sure to scrape down the sides of the bowl to incorporate any flour sticking to the sides, then transfer to the prepared baking pan and spread into an even layer. Set aside.
To make the cookie dough, cream the butter, sugar, and brown sugar on the high setting of a stand mixer for 3-4 minutes, or until it’s pale and fluffy. You’ll want to scrape down the sides of the bowl a few times to incorporate anything sticking to the sides.
Mix in the egg and vanilla, scraping the sides of the bowl as needed.
Add the salt, baking soda, and baking powder and mix until incorporated.
Pour in the flour, mix until nearly combined, and fold in the chocolate chips. Finish mixing by hand with a silicone spatula, scraping down the sides and bottom of the bowl to incorporate any sticking flour.
Scoop spoonfuls of cookie batter into the prepared pan on top of the brownie batter. We aren’t looking to spread it. The dollops will let the brownie part show through to the top for a nice effect.
Sprinkle additional chocolate chips on top, if desired.
Cover the pan with foil and bake for 20 minutes. Uncover and bake for another 20 minutes.
Let the brookies cool completely before removing them from the pan and cutting them into 15 even pieces. Enjoy!