My most recent cake decorating class was an introduction to modeling chocolate (or plastic chocolate, chocolate clay, etc) it is a pretty fun medium to work with and tastes good compared to gum paste but is very heat sensitive and doesn't dry as stiff as gum paste so if you need to use it in the dead to summer forget about it!
Recipe is super easy
1 pound of chocolate (brown or white)
1/3 c. corn syrup
to make just place both ingredients into a hot water bath and let the chocolate melt. (Don't let it get too hot!) stir occasionally and replace the hot water as needed (don't put on the stove or microwave) if it gets too hot it will have a grainy texture you can't get rid of.
once it is melted and stirred you can wrap in syran wrap and place in a ziplock to cool and harden if you want a softer chocolate paste.
If you want it to be stiffer (more like gumpaste or fondant) you will continue to stir until it comes together and the oils start to separate out---Pour these oils off and continue to stir (you may get up to 1/4 c oil off) the more oil you leave in the mixture the softer the mixture will be and the easier it will melt in your hands. Once you get as much oil off wrap in syran wrap and let cool ~1-2 days. (the modeling chocolate will now be HARD) cut off a chunk and the warmth of your hands kneading it will soften it. Roll it out like fondant and shape.
The modeling chocolate in a ziplock and tupperware will last several months and the pieces can air dry and will last at least several weeks to a month.
Con's: can't use in warm weather because will melt, can melt with the heat of your hands so pieces can loose their shape if handled too much.