For the peanut butter frosting, blend together peanut butter, powdered sugar, butter, and heavy cream in a stand mixer for 3-4 minutes or until light and fluffy. If it is too stiff, add a bit more milk so it is easy to pipe.
Using Wilton tip 233, pipe "hair" onto the top of a brownie in whatever way you'd like. Then use upside down Reese's Minis peanut butter cups to place eyes on your monster. Put a tiny dollop of white icing on a chocolate chip or Reese's Pieces to use as an eyeball.
If desired, use a decorating bag, small tip and white icing to pipe smile onto your monster brownie. Or you can leave them with just the eyes!