Hi, I'm doing a 3 layered black and gold 21st cake shortly and one of the layers is going to be completely gold. What is the best way to get a smooth, sparkly gold covering on top of fondant?  I've tried airbrushing but I can't get it to cover completely so it ends up a bit patchy  

First, make sure whatever you're using is food-safe and edible, not just non-toxic. I don't know many things that will make it sparkly that aren't inedible, but your best bet will probably be to use an edible gold spray, then dust over that with a gold powder like Crystal Colors to smooth out the finish. I just posted this somewhere else but I'll post it here too: http://acaketorememberva.blogspot.com/2015/10/metallic-and-pearl-luster-sprays-review.html

If yo're in New Zealand (I'm assuming from your screen name) There are a bunch of edible gold paints that you can get that are approved for food use outside of the US, too. I like the Rainbow Dust ones, but Rolkem makes some that people like.