what makes it crust? butter or shortening? thanks for responses.

You know I'm not exactly sure of the science behind it but I think it's the ratio of powdered sugar to shortening/butter. The more powdered sugar I use, the faster the cake crusts. A lot less powdered sugar and it won't crust at all. Meringue powder will also help icing crust.

..yeah, I was always under the impression that it was the meringue powder or flour that made the icing crust!