Before I attempt this how should I roll out black gumpaste? Do I use cornstarch? Or will thet leave me with white powder covering my decoration?
I have used a very, very fine sifting of cornstarch for gp. you can generally rub it out with your hands. Try a small piece, just to be sure.
for certain things, you can roll it on plastic without cornstarch, but not everything works that way.
I use white fat, just rub a small amount on your board and a litte on your rolling pin. I've never had any problems with it and it doesn't leave any marks.
I think that if you use corn starch you can just put a little shortening on a papertowel afterwards and it will take up the excess corn starch. It will make the fondant shiny for a little while but that will soak in and it should be perfect.
Looks like the cornstarch then the shortening afterwards did the trick! Thanks a bunch ladies!