Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch pan.
In a large saucepan over medium heat, melt 1 cup of butter. Remove from heat and stir in cocoa until smooth. Mix in the white sugar, eggs and 2 teaspoons vanilla. Mix in the flour, then fold in pecans and marshmallows. Spread evenly into the prepared pan.
Bake for 25 to 30 minutes in the preheated oven, until brownies start to pull away from the sides of the pan. Let cool in pan.
To make the frosting: In a medium saucepan, melt together 1/2 cup of butter and 1/3 cup of cocoa. Mix in confectioners' sugar and 1 teaspoon vanilla until smooth. Gradually beat in the milk, a tablespoon at a time until icing is a nice spreading consistency.