The diamond quilting can be done two ways (on buttercream) -- with a metal diamond quilter (Jennifer Dontz.com sells them) OR with an impression mat (Sugarshack sells them.) Make sure your buttercream is crusting or it won't work.
From Graceful Cake Creations
I was thinking it must be an artist's rendering, but then I looked at the website... might be real at that.
Here's an extreme closeup of one of their cakes with the same design.
I have used the diamond impression matt successfully on buttercream. Just let it crust first.