How to stop icing from running off iced buns?
I think you can find your answer here. Basically, when the ambient temperature of the surroundings is on the higher side, the icing melts since the butter cannot remain in a solid state. However, what can be done according to the link that I posted, you can add 2 tablespoons of cornstarch per 3 cups of icing to make the icing sturdier. Or you can use other ingredients like gelatin, or meringue powder which does the same as well.