Do you have a regular crusting buttercream recipe that you like? If so then you can add cocoa powder or melted chocolate to it. Just add until you get the taste that you would like. The only affect adding either to your recipe should have is making it drier. Just add extra liquid to the recipe to get the consitancy you need. I won't change the crusting factor.
I use this recipe:
but I melt (and cool to room temp) 6 oz. unsweetened chocolate. I usually need to add a little more cream but oh man this is GOOD!!
mine is
2,5 sups butter
2,5 cups shortening
splash vanilla
dump in cocoa till really dark
5 pounds PS
liquid ( which for me is powdered coffee creamer mixed 1:1 with HOT water)
HTH
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