Preheat oven to 350 degrees. Line a 9×9 baking pan with wax paper or foil and spray with cooking spray. (You can opt for a 13x9 pan if you want thinner bars.)
Combine flour, baking soda and salt in medium bowl. Set aside.
Beat the butter, sugars and vanilla in a mixer. Add egg. Mix.
Slowly add flour mixture into butter/sugar mixture and mix until just combined. No need to overmix.
Press cookie dough into prepared pan.
Cover your cookie dough with Oreo cookies
Make the Brownie batter according to box directions.
Pour brownie batter evenly over the Oreo cookies.
Bake at 350 degrees for 40 minutes.
Cool for 5 minutes and then enjoy! Add some ice cream to completely put this indulgence over the top!