1. If I want to make small hearts to place on cupcakes frosted with buttercream, will either work? Or is buttercream too soft to keep the hearts solid and dried?
2. If I want to cover cookies, how do I adhere the fondant heart to the sugar cookie?
Gum paste does harden faster than fondant. Either one would work, or a mixture of 50/50, my experience with fondant on buttercream was that after a while it did absorb moisture from the frosting and softened up. But if you were going to serve them right away, would probably be ok.
When covering cookies, you brush a little karo syrup on the cookies and then place your fondant on.
I've used gumpaste decorations on buttercream ..and not had a problem...but then I don't place them on the buttercream sooner than the day before I'll be delivering them.
Either of the tips you've been given work fine for attaching the hearts. Another option is to use royal icing.
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