How long do you need to wait? Overnight? If so, refrigerate the icing. Reconstitute it the next day by melting 1/3 of it in a bain marie. Scrape the rest into your mixer bowl, add the melted portion, and whip back together. Start on low speed, then raise it so you don't wind up with it flung about the kitchen. If you want to do it the same day, buttercream is stable at room temp for 4 hours, but don't forget to factor in how long it will be at room temp after icing but before eating the cake.
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