Preheat the oven to 350 degrees F. Lightly oil a 9x9 baking dish with cooking spray. Set aside.
To make the peanut butter cookies: Add softened butter, white sugar and light brown sugar to a medium mixing bowl. I usually slice the butter into a few pieces to make it easier to incorporate. Using a hand or stand mixer, cream butter and sugar together on low until fluffy. Add one egg, creamy peanut butter and vanilla extract. Mix on low until just combined.
Mix together the dry ingredients in a separate bowl: spoon and level all purpose flour (see note 2), baking soda, salt. Slowly stir this mixture into the wet mixture using a wooden spoon. Fold in the chocolate chips. Cover bowl and place in the fridge until ready to use.
To make brownies: Using a wooden spoon, stir together white sugar and cocoa powder in a medium mixing bowl. Once combined, add oil, two eggs, and vanilla, mixing until incorporated.
Stir in the all-purpose flour (using spoon and level method) and a pinch of salt. Mix until just combined. The batter should be wet but not liquidy.
To assemble: Add brownie batter to the lighlty oiled 9x9 baking dish. Use a tablespoon to scoop ping pong ball sized cookie dough pieces-- basically the size of a regular cookie dough scoop -- onto the brownie batter. They will sink into the batter a little bit, that's okay.
Bake at 350 for 25-30 minutes. 25 minutes yields fudgy, almost underdone brookies. 30 minutes will still be soft, but less so. Press a fork or toothpick into the center of the brownies to test for doneness before removing from the oven. Allow to cool before slicing for best results.
Oil: I use canola oil because it has a neutral flavor. A lightly flavored olive oil can be used in place of canola.
Spoon and level: For consistent measuring, take a spoon and scoop flour into the measuring cup until overflowing. Use the back of a knife to wipe excess flour off the top. Do not pack flour into cup.
Leftovers: Store cooled brookies in a closed container at room temperature for 2-3 days. Store them in the fridge for 4-5 days.
For harder cookie bars, bake for 35 minutes instead of 30.