Preheat the oven to 350 degrees. Spray a 9 x 9 baking dish with cooking spray.

Prepare the brownie mix according to the package directions. Add vanilla and salt if desired.

Bake for 30 minutes.

Remove from oven and sprinkle with marshmallows so that they are in an even layer.

Return to the oven for 3-5 minutes or until the marshmallows puff up.

Remove from oven and cool completely.

To make the frosting place the butter in a large mixing bowl and beat with a mixer on medium high speed.

Add in the powdered sugar and cocoa powder and mix for 1-2 minutes or until totally incorporated.

Add the milk and vanilla and beat on high for 2 minutes or until fluffy.

Spread the frosting on top of the brownies in an even layer.

Cut into 16 squares and serve.