I made fondant out of marshmallow, powdered sugar & shortening. At first i am satisfied but later on it become moisten. Whats the did i do? Is it better to use cornstarch than shortening? Please help.

I would use cornstarch on the rolling surface to prevent sticking. If your fondant is to moist it could be because you had to much liquid in the MMF mix. But if it was okay at first it is probably something else. Was your cakat room temperature when you covered it with fondant? If it was froze or really cold as it thaws it will make the fondant wet.

I used cornstarch on the bottom once and it stuck. I always use shortening. You probably had too much water in it. Just add more powdered sugar next time....

Where did you have the fondant stored? If it was in the fridge, that is why it got wet.