There are plenty of cold porcelain recipes out there, but I wanted to find one that would be totally non-toxic and would be okay to use for toppers, etc. I adapted one that had baby oil in it so that the ingredients were as food-y as possible, and it worked to make these little roses that were dry the following day.
Even with this recipe, though, I'd still make sure that there was some kind of barrier between the flowers or whatever it is that you make from it and the cake. A thin piece of gumpaste or fondant should be fine to prevent it from touching the cake.
This made a decent clay that was ready to be shaped by hand, but it wouldn't work in the molds that I have. It didn't want to come out of the mold because it was a little too wet. If you added a bunch of corn starch to make it stiffer it would probably work in a silicone mold, but I can't promise anything.
This doesn't need to be cooked, but I guess that you could heat it up then knead it until it's doughy. It was very wet when I made it, so I've adjusted the amounts on that. Add more as you need to if it's too dry.
No-Cook, Non-Toxic Cold Porcelain
1 1/4 cup corn starch
1 cup white glue
2 Tbsp baby oil
1 Tbsp lime or lemon juice
Add that ingredients together in a stainless steel bowl. Mix them up then knead the dough until it's a good moldable consistency. If it's too wet add more corn starch, and add some lime juice by 1/4 tsp amounts if it's too dry. When it's a consistency that you like, let it sit for a while before using it. Wrap it in plastic wrap to store, and you can keep it in the fridge for extended periods.
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA, and cake supplies online at www.acaketoremember.biz and http://www.acaketoremember.etsy.com/