I have made IMBC and absolutely love it! I used it to frost some cupcakes last Halloween. How stable is this frosting in the summer? Can it hold up to high heat or humidity? How long can cakes be left out frosted with this buttercream?
I've never made IMBC, but I've seen the recipe and I can tell you this: there's not such a big problem with spoilage, because the egg whites in it are cooked (although in the heat all bets are off!), but it does contain a lot of butter, which will make it melt, or at least go really soft in the heat.
i don't think an all-butter IMBC can hold up in heat well at all. i've seen other CC users say they substitute some of the butter with shortening and it holds up better. not sure if humidity is a problem ...
i've also heard CC users say IMBC holds up better in the heat than SMBC. Not sure why.
I live where it is 110+ degrees for weeks at a time in the summer. I exclusively use a meringue bc, just not IMBC. I wouldn't dream of using anything else, instead I educate the client that cake just doesn't belong outside during the summer. That's it, pretty simple. That said, I had a customer insist on taking a cake to the park in the middle of August even though I told her it might be a mess and I strongly discouraged her to do so. Turned out fine. I delivered chilled, they ate it within an hour of being delivered and had no problems. Actually, she said it was nice, cause the cake was super cold, and people were liking the ice cream cake effect it kind of had...weird.
But anyways, good air conditioning during your prep phase, a/c in the car, and understanding that it just doesn't belong outdoors period. Unless you're my clients apparently. Weirdos. Just kidding-they are lovely repeat customers.
I only make IMBC and I use 1cp butter and 1/2 cp shortening. I think it holds up quite well in the summer