i have a cake to make for a friend's bday party & although i adore the way colette peters' smbc holds up, she prefers the taste of american buttercream. i had a problem with this buttercream melting very quickly & i am nervous about making this bc for a cake that will be out for a while. what if the fondant starts to slide off? i read that cream of tartar will make the icing harden w/o changing the flavor. is this true? i'd really love to make collette's american buttercream for her cake, but i'm so nervous after the melting i experienced last time.

i am a beginner and any tips you have that would be able to share would be much appreciated. thanks!!!