I found this recipe, here. And I had to try it for myself.
Here is what you need.
Here is what you do.
Make your brownies per the package instructions. Let it cool completely.
Microwave the marshmallow fluff for 30 seconds, stir halfway through. Frost the brownies and let it cool.
Microwave the frosting for 10 - 15 seconds and then frost it like a cake.
You can either let it cool or jump right in. It's good either way. And it's good a couple of days later too!