I've always loved this type of brownie. The pillowy marshmallow topping makes them a comfort food for me. This classic recipe, with very slight modifications, is from Six Sixters' Stuff blog.
1 cup butter (2 sticks)
2 cups sugar
1/3 cup cocoa
1 1/2 cup flour
1 tsp salt
2 tsp vanilla
1 (10 oz) package of miniature marshmallows
Soften butter and blend in sugar and cocoa. Beat together and add 4 eggs, one at a time, mixing after each one. Add flour, salt and vanilla and mix well.
Spread on large (10 x 15 x 1-inch) greased cookie sheet. Bake 22-25 minutes at 350. Remove from oven and cover entire top with 1 package of miniature marshmallows. Return to oven for 3 minutes, until marshmallows are soft and puffy. Cool and frost with chocolate frosting.
1 stick butter (1/2 cup), softened
1/4 cup milk
1-2 tsp vanilla (depending on your taste)
3 tablespoons of cocoa
2-3 cups of powdered sugar
Mix all ingredients with a hand mixer until smooth. Add more powdered sugar or milk until you reach desired consistency and spread on top of bars.