Preheat oven to 350 degrees Fahrenheit. Line a 9x13-inch baking dish with parchment paper, leaving a 2-inch overhang on the sides, for lifting the bars out of the pan.
BROWNIE BATTER: Whisk together the sugar, flour, cocoa powder, baking powder, and salt.
Stir in the egg, melted butter, water and vanilla. Fold in the chocolate chips. Set the brownie batter aside while you make the cookie dough.
COOKIE DOUGH: Use an electric mixer to cream the butter with both sugars until light and fluffy. Add the egg and vanilla. Mix together and scrape the bowl.
Whisk together the flour, baking soda, and salt. Add the dry ingredients to the butter mixture and mix until just incorporated.
Fold in the chocolate chips.
ASSEMBLE AND BAKE: Spread the brownie batter into a thin layer in the prepared baking pan.
Drop spoonfuls of cookie dough into the brownie batter. May flatten the cookie dough slightly, but allow the brownie batter to peak through — do not cover completely. Sprinkle with extra chocolate chips, if desired.
Cover the pan with foil and bake at 350˚F for 25 minutes. Then remove the foil and bake for another 15 minutes.
Allow the dessert to cool completely before lifting it out of the pan with the parchment paper. Then cut it into squares for serving.
Notes
One 12-oz bag of chocolate chips is enough for both batters.
How to know when brookies are done baking: The cookie dough will be golden brown and a toothpick inserted into the brownies will be slightly gooey, but not wet. This can be pretty tricky to determine because of all the chocolate chips. The bake time is pretty accurate if you know that the temperature of your oven is precise. The bars will also continue to set as they cool.