I'm going to disagree with everyone and say it's WHITE CHOCOLATE!
If you look at these two cakes from CC member Denise, they look almost identical to your picture...http://cakecentral.com/gallery/1846835http://cakecentral.com/gallery/1329215
I had asked her a couple years ago how she made her boards, and this was her response...
"The cake board was covered in white chocolate - painted on with a Rubbermaid 2" paintbrush and when dry was painted with luster dust and Everclear. It makes a beautiful board and is pretty quick and easy."
Hope that helps! Plus it sounds a lot easier than the fondant!