Follow the directions and ingredients on the back of the brownie box.
Prepare a 9x9 metal baking pan with butter and parchment paper.
Pour the brownie batter into the prepared baking pan.
Combine the flour, salt, and baking soda in a medium mixing bowl.
In the bowl of an electric mixer, beat butter and both sugars.
Add in the egg and the vanilla extract.
Turn the mixer to low and add the flour mixture.
Scrape the bottom of the bowl and mix until the ingredients are combined.
You'll only need about 1/2 to 3/4 of the cookie dough so you can store the rest, freeze it or make cookies with it.
Scoop the cookie dough and place it directly onto the brownie mix. Do at least 12 giant scoops of dough to cover the top.
Bake the brookie for 40 to 45 minutes in a preheated 350-degree oven.
The brownies will be puffy on the edges and in the middle and the cookies will be golden brown.
Remove and let them cool for 10 minutes before serving.