Combine sugar and butter in an electric mixer; add eggs one at a time.

Add vanilla, cocoa, and flour; combine well.

Pour brownie batter into a greased 9x13 pan. Bake 23-25 minutes at 350°.

Remove from oven and cover with marshmallows. Place back in the oven for two more minutes, or until marshmallows start to get puffy. Remove from oven and sprinkle with chopped walnuts.

Time to make the frosting! Melt margarine in a saucepan over medium heat; add cocoa and milk. Bring to a boil and add powdered sugar. Drizzle over brownies while hot, allowing some of the marshmallows to show through.

Let cool before cutting. Enjoy!