Preheat oven to 350°. In a KitchenAid stand mixer, cream together melted butter, brown and white sugar.
Then mix in eggs and vanilla extract. Beat together.
After combining wet ingredients, add in cocoa powder and beat together until smooth. Add in flour and baking powder. Beat again.
Then, pour the brownie batter into a greased baking dish. Add melted marshmallow fluff and peanut butter to the top of the raw brownie batter and swirl. Bake for 18-22 minutes.
Top with more marshmallow fluff, peanut butter drizzle, and melted chocolate if you’d like 🙂 Enjoy!