Combine the 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 9×13-inch pan. Bake at 350 degrees Fahrenheit for 23 to 25 minutes.
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.
Make the frosting by melting the margarine in a saucepan over medium heat; add the cocoa and milk. Bring to a boil and then add the powdered sugar and vanilla.
Drizzle over brownies while hot, allowing some of the marshmallows to show through. Let cool before cutting. Enjoy!