I sure can think of a lot of reasons to put a tiny little brain on a cupcake; they'd probably especially be a hit with all the zombie aficionados out there. All you need to pipe brains are stiff consistency gray royal icing (I used the marbled gray icing from the headstone), round tip #12, oval tips #57 and #55, parchment paper squares, a glue stick, a flower nail, a Styrofoam block to rest the nail in, and a dusting pouch filled with a 50/50 blend of powdered sugar and cornstarch. Start by sticking a parchment square to the flower nail with a dab from the glue stick.
You might want to do an image search for brains; it's helpful to have a picture of one as a guideline. Hold the #12 tip just above and at a right angle to the surface and pipe a shape sort of like a bean. Add a little more icing to the hole in the center, if there is one.
Liberally dust a fingertip by tapping it to the dusting pouch, then tap the icing into this basic brain shape. Allow it to firm up for about 10 minutes.
Load the larger oval tip (#57) to the gray icing bag and pipe a bunch of squiggles all over the larger, upper part of the brain.