In a large bowl, blend the butter, cocoa, and sugar. Add 4 eggs, one at a time, mixing after each one. Carefully add flour, salt and vanilla and mix well. Spread brownie batter on large (10x15x1 inch) greased cookie sheet. Bake 22-25 minutes and let cool for a few minutes.
While waiting for the brownies to cool, melt peanut butter in the microwave (I microwaved mine for 60 seconds, pulled it out and stirred, then microwaved it for another 60 seconds. It was smooth and melted by that point). Spread melted peanut butter over warm brownies and let set up (stick them in the fridge to set up faster).
Once the peanut butter is set, make the frosting by combining the milk, cocoa, butter, and marshmallows in a medium saucepan. Cook over medium heat, stirring until the marshmallows are melted and the chocolate mixture is smooth. Remove the pan from the heat and carefully add the powdered sugar. Once the frosting is smooth, spread over the peanut butter layer and place brownies in the fridge for 30 minutes so they can set up.