On Mondays, Wednesdays and Fridays in March you can get the shamrock shake cupcake ("Shamrock shake is green vanilla cake topped with mint buttercream and a LUCKY RING") at Retro Bakery in Las Vegas. Every day, you can get their pistachio cupcake, which is "Pistachio is pistachio cake topped with pistachio buttercream and crushed pistachios." Visit retrobakerylv.com for more information.
shamrock shake cupcake via Flickr
they also do custom cupcakes like these painter cupcakes! order at least 14 days in advance