I have a wedding coming up in June that will take place outdoors in a barn. The theme is rustic elegance and the bride likes the look of the "messy-iced" cakes that are becoming popular in part, because of Pinterest. I've attached a picture to show something similar to what we're doing.
http://www.stylemepretty.com/gallery/picture/515974

Temperatures in an Oklahoma June can exceed 100 degrees and for a 7 o'clock wedding I'm anticipating at least 85 degrees in the barn. What is my best chance of getting a stable buttercream that will be able to take the heat and humidity for the longest amount of time? She expressed that she doesn't want the traditional American buttercream containing shortening. Would SMBC be better than an all-butter buttercream or what about white chocolate ganache? She wants the cake to be ivory in color, not white.

The groom's cake will be a tiered chocolate cake and she likes the idea of chocolate curls or even big "shards" of chocolate on the outside of his cake. I'm worried about condensation and slumping of the chocolate. Any tips to avoid this would also be appreciated.

I'm used to working with fondant and I've certainly never done buttercream for an outdoor event. Thanks in advance!

I think your best bet would be an american buttercream made with shortening or a combination of butter and shortening. An all-butter based buttercream may work okay since you don't need sharp lines. It would be very soft but probably wouldn't literally melt. I'd be suprised if SMBC holds up better than standard american buttercream since it's made with all butter, but I've heard some people sub in some shortening with success. Chocolate ganache would probably be about the same consistency. 85 is just a tough temp to work with! If I were you I'd make a few mini cakes and try to replicate the conditions to see what happens.

Buttercream cake in heat:

I would try to convince the bride to get a fondant cake, if not definitely go with shortening based buttercream and get her to sign a waiver that you are not responsible for heat related damage. Good luck.

Charlotte's Whipped Cream Buttercream will hold up. I use it all the time for the last decade. Inside, outside, any temperature.

Google it.

The cake I did for a barn wedding was in the fall so I didn't have to worry about the heat but It turns out the floor of the barn was a little shaky and everytime someone walked past the table shook. Fortunately my cake supports held- just a word to the wise. Good luck

Thanks for your suggestions, everyone. Maybe my best bet would be just to have the cakes frozen solid. How long would it take them to thaw in those conditions? I'm assuming that they'll probably be sitting for ~2 hours prior to the cutting.

Leah: I was getting excited reading Charlotte's recipe until I got to the part where you add 1 lb. of shortening. The Bride mentioned that she was grossed out at the thought of shortening in her icing. That's my big dilemma.

FullHouse: After watching Bronwen's video I'm literally horrified!

Freezing the cake would probably lead to issues with air bubbles (due to gas releasing as it went from cold to hot) and condensation. I would warn the bride of the potential for issues with an all butter buttercream in a non climate controlled environment. Maybe even send her a link to that video. If she is still set on it, I would be sure to have her sign a waiver that you have explained the risks and she assumes full responsibility for any and all weather related damage to the cake.

Let us know how it works out.

Good point about the air bubbles. I do have a waiver in place and I did let her know that someone would have to sign off on it when I deliver it. I've warned her about the risks involved and I emailed her today letting her know that I could let her sample the shortening-based icing. I told her that I just want her to be happy with it and I want it to look great until it's cut. I think she just had previous experience in which a friend's cake didn't taste great and when they asked what the frosting was made of someone told her Crisco so she was immediately grossed out. Hopefully I can convince her otherwise. She's a pretty easy-going bride so I think it'll work out fine.

I have done dozens of b'cream iced cakes in CA w/o problems. One especially was set up around 1 in the afternoon *in full sun - outside* on a hot summer day (dont remember the temp now) and if anything dried out rather than melted.
I use a recipe that is 1/2 butter and 1/2 shortening.