Preheat oven to 350°F. Spray a 9x13" baking pan with nonstick cooking spray. Line with parchment paper with an overhang on the sides and spray again. This will make it easy to lift the brownies out of the pan after baking. Set the pan aside.

In a large mixing bowl stir together melted butter, granulated sugar, brown sugar, eggs, and vanilla extract until smooth and combined.

Sift in flour and cocoa. Add the salt and espresso powder, if using, and stir until just combined. Don't over mix. Fold in the chopped chocolate.

Chop up 15 of the Oreos and stir them into the batter. Spread evenly into prepared pan. Roughly chop up the remaining cookies and place them evenly on top of the brownie batter.

Bake for 25 to 35 minutes on the middle rack in the oven until the brownies are just set to touch. Don't over bake the brownies. They will set up as they cool. Remove from oven and sprinkle with flaky sea salt, if desired. Let cool completely before cutting into squares.