Preheat oven to 350°F (175°C). Grease a 9×13 inch pan or line it with parchment paper that extends above the sides for easy brownie removal. Set aside.
In a microwave-safe bowl, combine the butter and semi-sweet chocolate. Heat in 20-30 second increments, stirring after each, until fully melted and smooth.
Add the sugar and whisk until well combined.
Whisk in the eggs and vanilla extract until smooth.
Gradually stir in the cocoa powder, flour, and salt until the batter is thick and fully incorporated.
Fold in the chopped Oreos.
Spread the batter evenly in the prepared pan and bake for 25-30 minutes. Check for doneness by inserting a toothpick into the center; it should come out with a few moist crumbs.
Let the brownies cool while preparing the cookies and cream layer.
In another microwave-safe bowl, melt the white chocolate chips with the heavy whipping cream in 20-30 second increments, stirring after each until smooth. The mixture will be thick.
Stir in the powdered sugar and chopped Oreos. Spread the mixture evenly over the cooled brownies.
Allow the brownies to cool completely before cutting into squares. The top layer will be firm but perfectly complements the fudgy brownie base.
Store leftovers in an airtight container at room temperature for up to a week.