Preheat oven to 350° degrees F.
Whisk together melted butter and cocoa powder. Stir in sugar. Add in beaten eggs and vanilla extract and mix well.
Stir in flour, salt and chopped pecans.
Spread batter into a greased 13X9-inch pan.
Bake at 350° for 30 to 35 minutes or until toothpick inserted in center comes out clean.
While brownies are baking, make the chocolate frosting: Using an electric mixer, beat together powdered sugar, cocoa powder, softened butter, vanilla extract and milk.
Remove brownies from oven. Stir marshmallows into chocolate frosting then immediately spread over hot brownies.
Let the brownies cool completely before cutting into squares.