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! :)
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.