Preheat the oven to 350 degrees.
Line a 9x13 baking dish with parchment paper, or lightly spray with cooking spray and set aside.
To make the peanut butter cookie dough: Add softened butter, white sugar and light brown sugar to a medium mixing bowl.
Cream butter and sugar together on low until fluffy.
Add one egg, creamy peanut butter and vanilla extract, and beat together.
Mix together the dry ingredients in a separate bowl.
Add dry ingredients to wet mixture, and combine well. Set aside.
To make brownie batter: Combine brownie mix with eggs, oil, and water
Mix until well combined.
To assemble: Add brownie batter to the prepared baking dish.
Spoon peanut butter cookie dough over brownie batter
Sprinkle with chocolate chips if desired.
Press down a bit if the cookie dough is crumbly to get a more cohesive bar. It’s okay if it sinks into the brownie batter a bit.
Bake at 350.
After 30 minutes, test the center of your brookie bars for doneness with a fork or toothpick. If the brownie underneath seems too gooey for your liking, bake another 5 minutes, then test again.
Once brookie bars have reached desired doneness, remove from the oven and let cool in the pan for 15 minutes or so before slicing and serving.