Stir together butter and sugar. Add the eggs and vanilla and mix well. Mix in the flour, cocoa, and salt. Pour into a greased 9x9 (if you want them really thick) or 9x13 pan. Bake at 350 degrees for 20-25 minutes (a few more if it's a 9x9--check often), or until toothpick comes out clean. Don't overbake! Remove brownies from the oven, then evenly cover the top of the brownies with the marshmallows. Place bake in the oven for 5 minutes.
While the brownies are baking, make your frosting! Mix the powdered sugar and cocoa together, then beat in the butter and add milk unto you get the right consistency. Add salt, mix well. Microwave the frosting for about 30 seconds, or until it can be easily poured. Stir in the pecans, then pour over the marshmallows.
Cool before cutting. If you can! :)