Q. I need to make some rolled fondant for a friend's baby-shower cake. We both keep kosher so the fondant needs to be kosher. The recipe calls for 2 tablespoons of gelatin. I have a box of unflavored diet Kojel jel dessert. Can I substitute the jel dessert for gelatin?
Who's gonna know? Fondant, a sugar paste, is rolled thin and used to cover cakes. It makes a beautiful, pure white surface that you can decorate and helps the cake retain moisture. The gelatin is an ingredient that makes the fondant elastic and helps keep it supple so that it doesn't dry out and crack (some recipes also use a touch of glycerin for the same purpose).
We don't know whether your Kojel-brand dessert mix has the same relative strength as animal-based gelatin, so can't say whether you should substitute it on a one-to-one basis. But we doubt some variation will matter that much to the texture of your fondant. If it is not perfectly clear or white, however, it could mar the bright white color that is one of the primary reasons you're making fondant in the first place.
Now, to be brutally blunt, fondant doesn't taste very good it's basically a layer of uncooked confectioner's sugar. The use of diet Kojel could impart a somewhat bitter aftertaste to something that is already a little dicey. But if you're accustomed to consuming foods and drinks made with artificial sweeteners, you've probably already come to terms with the aftertaste issue.
The key to using fondant, and frankly, the only way you can get away with it at all, is if the cake and any other fillings, frostings, and decorations are all so delicious that they completely overwhelm the taste of the fondant.