Buttercream will crust depending on the powder sugar to fat ratio. Buttercream is the only frosting that I ever use. Maybe you are looking for a meringue type frosting (I don't think those crust).
Go to YouTube and look up Warren Brown's IMBC recipe. I love it. I also use this recipe which you can use with butter, shortening, or both:
http://cakecentral.com/recipes/2019/whipped-cream-buttercream-frosting
Look up the Whimsical Bakehouse recipe.
Crusting does have to do with sugar to fat ratio. If you want the recipe you use to crust less, you have to either up the fat, or reduce the sugar in it.
Yes, more of what every 'fat' you use in the recipe, be it butter or Crisco.
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