Preheat oven and follow the brownie mix instructions.
Bake at 325°F for 24-26 minutes. Depending on your oven, maybe check at 24 minutes with a toothpick. If it comes out clean they are done, if not continue baking. I preferred them a little underdone and fudgy, so when I pulled the toothpick out it was slightly wet. They will still cook after removed from the oven.
For the frosting: In a mixing bowl cream together the butter and powdered sugar. Add the peanut butter and mix until smooth. Add the marshmallow cream and mix until smooth.
Fill a piping bag with tip of choice and pipe in circular motion on top of each, cooled brownie cupcake.