Stir in the flour, cocoa powder, salt, and vanilla then pour into a prepared 13x9 inch pan.
Bake for 25 minutes.
While brownies are baking, make the frosting: Stir cocoa powder and milk into melted margarine. Bring to a boil and cook for 5 minutes - stirring constantly. Pour into the mixing bowl with powdered sugar and beat until smooth.
Remove brownies from the oven after 25 minutes and cool for 10 minutes.
Spread marshmallows over top and pour the frosting on top of the marshmallows