That is definitely an American buttercream, a combination of butter, shortening and powdered sugar.  I like seriouscakes recipe, www.seriouscakes.com.  You don't need a crusting buttercream, but definitely an American buttercream.  If it is going to be outdoors, you'd want at least 1/3 of your fat to be hi-ratio shortening, like Alpine or Sweetex. 

You could do that with Swiss Meringue Buttercream, but if it gets hot it will not be very stable.
 

Thanks for the quick reply!  I will have a look at that recipe! :) 

I put meringue powder on my buttercream if the cake is  going to be outdoors  and used water or extra flavoring instead of milk...works wonders! :)

Thanks :) 

AIt doesn't need to be crusting. Those rosettes are a lot of icing and I think that would be too sweet. PM me and I will send you my recipe. It's pretty heat-friendly. You should definitely keep the cake cold for as long as you can before putting it under the tent.