Preheat the oven to 350°F. Spray a 8x8 or 9x9 square pan with oil spray.
In a stand mixer bowl or with a handheld mixer and bowl, add butter, peanut butter, brown sugar, and granulated sugar and cream together on high speed.
Decrease speed to medium and add eggs, one at a time, then vanilla, beating well between each.
In a separate bowl, mix together cocoa powder, flour, and salt.
Add dry ingredients to wet ingredients, gently stirring together. Fold in chocolate chips.
Pour batter into prepared pan and bake 15 to 20 minutes until the top feels firm and a toothpick in the center comes out clean.