AWill a traditional buttercream icing still crust if all butter is used instead of shortening?

I think mine does, and it's always been all-butter. But being a dense, hand-mixed, cold-process buttercream, it's kind of hard to tell. But it definitely resists being pulled away by aluminum foil over the cake in the refrigerator.

 My strawberry frosting, probably because of the amount of jam in it, definitely doesn't.

Yup, the All American Buttercream recipe on this site crusts. Moreso if you put it in the fridge :)