Did you look in the Recipe here in CC? There are many good recipe and all of them are with cocoa powder, I'm not sure but maybe you can substitute some of your powder sugar with cocoa powder and make you choc buttercream.
I'm not an expert maybe someone else can help.
Good luck
Wendy
to our standard buttercream, we add cocoa, hersey syrup and sometimes some chocolate extract. Just take some of your mixed up vanilla and start from there. 
If the icing is too thick you can alway add a little more milk or water, whichever you use in your original recipe.
we always use butter and alpine shortening in our recipe, so it it great.
hope this helps.
Has anyone experimented with how to best get a chocolate bettercreme? My DH LOVES the vanilla and his b/d is this week and I would like to make his cake with chocolate instead. tia
Bettercreme comes in Chocolate. I'd check where you buy the vanilla and see if they carry it.
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