Can anyone tell me the best way to achieve this look in buttercream.  I'm making this cake for my niece and I know it won't be exactly the same but what tip would you use to get a similar affect in buttercream?  Tia 

Hmmm....I can't think of many piping tips that would make that exact shape (but maybe I'm just not knowledgeable enough). But, with a 104 ruffle tip you could create somewhat of a similar effect, especially depending on how you make the ruffles (shorter swoops instead of longer ones). 

Here's a youtube video using a 402 tip that is also quite similar! Maybe that will get you closer to the look. 

Very helpful thank you!

It won't be the same but close if you use a large rose tip - (maybe 127?) and pipe row after row of short ruffles undulating around.  Graduate the color and I bet it will be just beautiful.