You may want to try this recipe for Crusting Cream Cheese BC:
http://www.cakecentral.com/cake-decorating-ftopict-1541-.html
This is the cream cheese frosting I use--its not buttercream but it crusts:
-1 stick unsalted butter softened--(margarine is too soft)
-10oz cream cheese softened
-4c powdered sugar
-1Tbsp milk
Cream cheese and butter, add sugar and milk.
I let this sit on the cake a little while and it crusts--15-25minutes. If you are wanting to decorate with it I usually omit the milk so it is a stiffer icing.
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