Make the brownies:
Preheat the oven to 350ºF. For thick brownies, spray a 9×9″ pan with non-stick spray or line with parchment paper. If you’d like to make thinner brownies, use a 9×13″ pan.
Prepare the brownie mix according to the package directions. Then pour batter into the prepared pan. Bake for 30 minutes, or until almost done. Do not overcook. Remove from oven and then sprinkle 2 cups mini marshmallows over the top so that they are covering the brownies in an even layer. Bake an additional 3 minutes, or until the marshmallows have puffed up. Remove from oven and allow to cool.
Make the frosting:
Combine 1/4 cup salted butter, 1 1/2 cups powdered sugar, 2 Tablespoons cocoa powder, 2 Tablespoons milk and 1/2 teaspoon vanilla in a large bowl. Beat with electric beaters until smooth. Add more milk or powdered sugar if needed to reach a smooth and creamy frosting. Then place spoonfuls on top of the marshmallows once cooled. Gently spread evenly.
I like to cut the brownies in 5×4 rows with a plastic knife.