Preheat oven to 325 degrees and line a glass 9x9 inch baking dish with parchment paper. Spray with cooking spray.
Place oreos in a ziploc bag and crush into pieces (not fine crumbs!).
Prepare brownie batter according to instructions.
Fold in half oreo pieces.
Pour into pan and bake 25 minutes, allow to cool completely.
Heat buttercream (either in a microwave for 20 seconds or on the stovetop on medium heat) until it is pourable.
Pour over top of brownie and sprinkle with remaining oreo pieces.
Melt white chocolate over a double boiler and spoon over oreos.
Swirl with the back of a spoon and refrigerate at least one hour.
Cut into squares and serve.