The best thing to do with boxed brownies...

I love these brownies.  They make me so happy.  I used Ghiradelli brownie mix, or use your favorite and throw in some chocolate chips.
THEN,
near the end, throw on some marshmallows, enough to cover all of the brownies.  Place back in the oven for a minute or two (make sure not to overbake the brownies though!) until slightly melted and soft.  
Let cool for a few minutes, and then add your icing.  You can use any chocolate frosting; I used the leftovers from my Malted Chocolate Cake.  I microwaved until soft enough to pour.  Or, you can use a canned chocolate frosting microwaved until really soft or this recipe: 

•  1/4 cup butter
•  1/3 cup baking cocoa
• 2/3 c. milk 
• 16 oz. powdered sugar

Combine all in a medium saucepan. Bring to a boil over medium heat, stirring constantly with a wire whisk, and boil one minute. Remove from heat and stir in powdered sugar. Immediately pour chocolate mixture over marshmallows. Let cake cool completely on a wire rack. Cut into squares.