Anyone have any advice on using IMBC or regular buttercream for an outdoor party this weekend? I'm partial to the look and the taste of IMBC but i'm wondering if it will hold up well enough in 90 degree heat. I'm not sure how long the cake will sit before we will cut it. It is a stacked cake and I'm using SPS.

Any suggestions would be greatly appreciated!

The as an all butter icing, the IMBC will melt in 90F heat. You need a shortening based buttercream. The only way you could get away with it is if it was kept indoors in air conditioning until just before it needed to be cut. It can't sit outside during the party.