Gather all ingredients.
Preheat oven to 325°F. Spray a 13- x 9-inch baking pan with baking spray; line bottom and sides with parchment paper, leaving a 2-inch overhang on long sides.
Prepare brownie mix according to package directions in a large bowl; spread evenly in prepared pan.
Beat butter, brown sugar, and granulated sugar in a large bowl with an electric mixer on medium speed until light and fluffy, 2 to 3 minutes, stopping to scrape down sides as needed.
Add egg, egg yolk, and vanilla; beat on medium speed until well combined, about 15 seconds.
Add flour, salt, baking powder, and baking soda; beat on low speed until nearly combined, 30 seconds to 1 minute.
Fold in chocolate chips just until combined.
Spoon cookie dough evenly over brownie batter in pan. (It’s OK that brownie batter peeks out in spots.)
Bake in preheated oven until cookie layer is golden and edges just start to pull from sides of pan, about 35 to 40. Gently press more chocolate chips into top, if desired. Let cool completely in pan on a wire rack, 1 to 2 hours.
Using parchment paper as handles, remove from pan and cut into 24 bars.
