I have never really experimented with a buttercream transfer, but I have a couple of other ideas for you. You can see if you can find a cute color picture of Barney(look online or in a book)...then see if you can have an edible image made. I know HEB and Kroger will sell the images by itself.
The other thing would be to find a picture that you like out of a coloring book and do a gel transfer. The way that I do it is I frost and smooth my cake first and put it in the refrigerator for about 15-20 min. Then I gel my pattern and put it on the cake. It goes on easy every time. Then you can trace your pattern and fill it in with tip 16 or 18 stars. Hope you got all that!!! Good luck!!! 
traci
And if none of those ideas work....You could see if a grocery store bakery could sell you the barney cake kit. I know that kroger will sell the kits for 4.00. You might call your local grocery store bakery before going there! Anyway, I hope it all goes smooth!
Traci
Here are some instructions for making a BCT...
http://cakecentral.com/article12-How-To-Create-a-Frozen-Buttercream-Transfer.html
http://www.webmall1.com/sweetdreams/frozbutter.htm
You can use wilton piping gel and color it or you can buy the kind in the tube. If you refigerate your cake so the icing gets real firm on top you do not even have to trace your picture on wax paper. I trace mine directly on the copy paper or coloring book page and place it on top of the cake and then take a small paint brush and gently go over the pattern. I have never had a problem with the paper sticking to the cake because I keep the cake in the refrigerator long enough for the icing to get firm. I hope that makes sense. If you have any other questions, just let me know!
traci
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