I understand that cornstarch keeps the fondant or MMF from sticking the the surface when you roll it out. That part I get. But why cornstarch as opposed to, say powdered sugar? Wouldn't slight amounts of the cornstarch stick to the fondant and make it taste funny?
I plan to experiment with MMF for the first time for my daughters 2nd birthday cake this June, and I am learning as much as I can now.
okay, while I am at it, is there a forumula to cake size and size you roll your fondant/mmf out to?
You can use cornstarch or powdered sugar if you wish. What you roll your pastes out on is a matter of personal preference.
Some folks use cornstarch or powdered sugar, but I find it very drying to the paste, often making it difficult to work with. If you are making homemade fondant and knead it on powders, you might find that it turns into a hard lump. If you color your paste, you will find streaks of powder on your paste when you go to apply it to the cake.
Many of us already use, or are switching to using Crisco on the work surface. It does not change the texture of the paste, nor does it leave powder streaks on the paste.
Theresa 
I have used a mixture of cornstarch and powdered surgar (equal parts of each) when working with fondant. Put it in the center of a cheap kitchen towel (the ones that aren't any thicker than a paper towel) gather ends of towel and secure with a rubber band. It looks like a little ball with a tail. Use like a powder puff and dust the table before you put the fondant down. Hope this helps.
I use cornstarch because it doesn't dry out the fondant as readily as powdered sugar. I also use less of it than powdered sugar. Cornstarch is slippery, whereas sugar is more likely to be absorbed by the fondant. That is my experience, anyway. And I've never noticed a weird taste. HOWEVER -- when I cut out pieces of fondant for wedding consult tastings, I always dust with sugar rather than cornstarch, since it will be eaten by itself.
It's been noted that either can be used, but keep in mind that corn starch is a thickener which means it will absorb moisture and make the fondant thicker/dryer.
I use powdered sugar for fondant. Sometimes I mix cocoa with powdered sugar in a new knee-high hose for chocolate fondant. I also roll my fondant out on heavy cotton taped to the counter instead of using plastic. This is just personal choice. Shortening on the plastic hasn't worked well for me. Maybe it's just the user. (Meaning myself, of course)
HTH
Michele
I used Crisco & it made my fondant shiny & greasy. Obviously, I used too much...but the fondant kept sticking to everything & I couldn't do anything with it.
I wish I had time to practice more w/fondant, because my only experiences w/it have been horrible.
That's why I do everything w/BC
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