Mix together melted butter, sugar, cocoa, eggs, vanilla and salt. Add flour and mix just until combined.
Bake in a greased 9×13 pan at 375 degrees for 25 minutes.
Immediately top with marshmallows and bake an additional 2 minutes. Use a knife dipped in water to spread marshmallows evenly. Cool completely, then frost.
Frosting
Whisk together ingredients until smooth. Spread frosting over marshmallow layer. Slice and serve.