Can I use cornstarch in fondant icing?
If you google "vegan fondant recipe", you will find a number of recipes for fondant which do not use gelatin.

In general, they use agar agar in lieu of gelatin (most seem to use flaked agar agar substituted one to one by volume for gelatin powder), and include glycerin as an anti-cracking agent.  Most seem also to use some hydrogenated vegetable shortening.

While it is possible that a cornstarch gel might be effective, I have not tried this, and none of the references I could find mentioned it&mdash;they all seem to use agar agar. 

. . .

If you are not averse to a commercial product, the Satin Ice prepared fondant is certified kosher pareve, and vegan, and so contains no animal products.  There may be acceptable commercial products, but this one came up several times in my research.