Preheat oven to 350°. Mix the butter, sugar and cocoa together until fluffy. Add the eggs, one at a time, beating after each one. Add flour, vanilla, and baking powder. Add walnuts if you choose.
Spread mixture into a medium-sized cookie sheet or jelly roll pan that has been greased. Bake for 25 min.
Remove from oven and cover with the mini marshmallows.
Put back in the oven for exactly 3 minutes, not longer or you will ruin them. Cool for one hour and then frost with the following frosting:
Frosting Ingredients:1/3 C. cocoa, 1/2 C. butter softened, 1 2/3 C powdered sugar, 1/3 C. evaporated milk Mix together and spread over brownies. Let set at least 12 hours before cutting.