i use Toba Garretts chocolate plastic which i love and has never failed me:
1 pound semi-sweet (dark) chocolate
5 fluid ounces (7.5 oz or 210 grams by weight) of light corn syrup...this is about 2/3 cup
1. Chop chocolate finely and place in a bowl over simmering water. Stir to melt chocolate evenly. When three-quarters melted, remove from heat. Continue stirring until all the pieces are melted.
2. Use a rubber spatula to stir in the corn syrup. Continue to stir until the chocolate starts to leave the sides of the bowl (about 60 seconds for dark chocolate, 20 to 30 seconds for white or milk chocolate).
3. Scrape chocolate mixture onto plastic wrap and spread out to about 1/2 inch thick. Place another piece of plastic wrap directly on top. Refrigerate or let rest in a cool place for 24 hours.
4. Once aged, cut into smaller pieces. Microwave the pieces for just a few seconds to take off the hard edge. Knead thoroughly with the heels of your hands until the chocolate has elasticity and a shiny coat. Wrap in plastic wrap until ready to use.
Keeps for several weeks without refrigeration provided it is placed in a cool, dry area.
For white or milk chocolate, use 1 oz less than you would for dark chocolate. Thus, use 4 fl. oz corn syrup or 6 oz/168 grams by weight for 1 lb. white or milk chocolate.
here are some more tips, most learned the hard way
....most recipes you find are going to have just about the same relative porportions of chocolate to syrup. i usually cut this recipe in half. there are also some good recipes on CC.
....milk choc/white choc (and candy clay i find) all have more fat, so you will want to adjust the recipe a bit as she mentions. you may also need to blot fat...el yucko. start with the semi-sweet, lol.
....a lot of recipes don't have the aging and i think this is important.
....easier if you let it come to room temp instead of microwaving before kneading because it is very easy to overheat and then you have to rip off a new piece and start over again because if it overheats it will get greasy, grainy, and floppy. don't throw the too warm stuff away. just tuck it back in fridge for later use.
...just knead until glossy. then STOP.
...i put the chocolate decorations right on top of my smbc and they stay very pliable (and tasty). my 5 year old peels them right off and eats them if i don't run interference...
....roses left out to dry will get hard...not hard like gumpaste, but hard enough to keep their shape.
....i have used petal dust on white choc flowers successfully.
ok, that's all i can think of for now
oops, one more thing (sorry, i did not mean to write a novel).
here is a link to a website where the decorator/chef uses modeling chocolate for just about everything, including covering the entire cake).