
Preheat oven to 350°F. Line a 9×13-inch pan with foil and spray it with cooking spray.
In medium bowl, stir together marshmallow fluff, mint extract, and green food coloring; set aside.
Microwave chocolate and butter in a large microwave-safe bowl for 90 seconds or until butter is melted. Stir until chocolate is completely melted.
Stir in sugar, then incorporate eggs, vanilla, and optionally espresso or coffee. Mix in flour until just combined.
Hand-crumble 12 Oreos over the bowl and fold them in.
Spread brownie batter in prepared pan. Drop about 1/2 cup of green filling onto brownie batter and reserve the remainder. Swirl with a knife.
Bake for 23 to 25 minutes or until brownies are set.
Cool completely, about 1 hour, or place in the freezer for 10 minutes if rushed.
Make vanilla frosting by whisking together butter, confectioners’ sugar, vanilla, and milk until smooth.
Melt white chocolate chips and add to green filling mixture. Spread over cooled brownies.
Sprinkle with 18 hand-crumbled Oreo Cookies, gently pressing them in. Allow to set before slicing.