I've only seen this done on fondant covered cakes. Wilton makes a tool for quilting on fondant.
Try Sharon's store
http://www.sugaredproductions.com/_product_85696/Diamond_Impression_Mat
I believe this is what you are looking for.
It can be done on BC. Sharon Zambito covers it I think in her BC DVD. Not sure about that though. Seems to me you just wait for the bc to crust and then use an impression mat.
Let the buttercream completely crust. Impress the cake with the diamond impression mat. Wipe mat off btw ea press. The diamond impression will more than likely not match up, but you can use where it meets as the back of the cake. It's very easy and people are so impressed with it.
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