Preheat oven to 375°F
Cream the butter and sugars until well combined. Add in peanut butter until well combined. With mixer on low beat in egg and vanilla until smooth.
Sift flour, baking soda and powder, and salt into a large bowl. With mixer on low slowly add in dry ingredients and broken oreos until just combined. Pour cookie dough onto a rimmed cookie sheet and press to about ½ inch thick, doesn’t have to be totally flat or reach edges of cookie sheet, just spread out.
Prepare brownie mix according to package directions. Take 3 tablespoons of brownie batter and drizzle evenly over cookie dough. Take a toothpick and gently swirl brownie batter into cookie dough only a few times. You should be able to see clear brownie swirls.
With a medium cookie scoop, scoop cookie dough with brownie swirls onto a parchment or silpat lined baking sheet. Bake for 9-11 minutes or until cookies are cooked through but still soft. Let cool for 3 minutes on baking sheet before transferring to a cooling rack to cool. Serve and enjoy!
**Save and bake the remaining brownie batter after cookies are done baking at 350°F according to package directions***.