In a large bowl, mix together the butter, cocoa, sugar, eggs, vanilla, and salt until well combined. Add flour and stir just until combined.
Spread in pan and bake for 20 to 25 minutes. Remove from oven and immediately top with marshmallows.
Bake an additional 2 minutes until marshmallows are slightly puffed.
Using a knife dipped in water, spread marshmallows evenly over brownies. (I keep a glass of water nearby and dip the knife in every so often so the marshmallows don't stick.)
Cool completely.
For frosting, combine frosting ingredients in a large bowl and mix with a hand mixer. Spread over cooled brownies.
Recipe by Simple, Sweet & Savory at https://www.simplesweetsavory.com/gooey-marshmallow-brownies/