Preheat oven to 350°F. Line a 9×13-inch pan with foil and spray it with cooking spray.
In a medium bowl, stir together marshmallow fluff, mint extract, and green food coloring until well combined. Set aside.
Microwave chocolate and butter in large microwave-safe bowl for 90 seconds or until butter is melted. Stir until chocolate is completely melted.
Stir in sugar until combined.
Stir in eggs and vanilla, adding espresso if desired.
Stir in flour until just combined, avoiding overmixing.
Hand-crumble 12 Oreos into the batter and fold them in.
Spread brownie batter in prepared pan, smoothing the top.
Drop half of the green filling mixture onto the brownie batter, swirling it through.
Bake for 23 to 25 minutes until set but fudgy.
Cool completely, about an hour or speed up by placing in the freezer.
Prepare frosting by whisking together butter, powdered sugar, vanilla, and milk.
Melt white chocolate chips and mix them into the frosting.
Spread frosting over cooled brownies and sprinkle remaining Oreos on top.
Allow to set fully before slicing and serving.
