Ok so I went to a birthday party and the cake they had was really good, it frosting wasn't too sweet. I've never tasted it before, but the frosting was really soft, like the cool whip type but a very very thin layer crusted hard. What do you think they added?

If there was a thin layer of crusted *hard* it sounds like a boiled icing or royal w/glycerin added. BUT........we could all guess all day long and probably never come up w/the right answer. You need to ask whoever made/brought the cake.

It sounds like the premade whipped frosting most grocery stores use.