Preheat oven to 350 degrees. Lightly grease a 9 x 13 baking pan. Use a 9×9 pan if you want thicker brownies!
Melt the butter in small saucepan until all the lumps are set aside. Whisk together the cocoa powder, hot cocoa mix, eggs, sugar, and vanilla. Once combined, add in the melted butter.
Using a wooden spoon, add in the flour, baking powder, and salt mixing until just combined. Fold in the mint chocolate chips. Spread the batter over the pan and bake for 30-35 minutes until a toothpick inserted into the center comes out clean. Remove from oven and let cool.
Beat the butter in your stand mixer until smooth. Add in the marshmallow fluff and continue to beat until smooth - be sure to scrape the sides of the bowl well!
Add in the powdered sugar 1/2 cup at a time to make sure it mixes well! While mixing, add in the vanilla and salt.
Once combined, add in heavy cream as necessary to create a smooth and creamy texture.
Spread the marshmallow frosting over the cooled brownies in an even layer.
If desired, melt the mint chocolate chips in the microwave in 30 second intervals to drizzle over the brownies. Sprinkle with crushed peppermints & mini cocoa marshmallows for extra decoration & enjoy!