Healthy cupcakes! Yay!

Does anyone know what icing sugar is or where you can find it?  I've never heard of it.  These Chocolate avocado cupcakes with avacado icing look interesting.  I like that the icing is green without using any dye.  (I know, avacados are healthy all on thier own, but hey, if you're gonna eat a cupcake anyway, why not a somewhat healthy one??)