Ok so I am going to make petit fours dipped in the chocolate/heavy whipping cream combo I saw a thread for on this site. Should I decorate the top of them with royal icing or buttercream?

Thanks!

Make sure the petit fore icing is dry, so the decor won't slide off. you can use buttercream. Personally, I wouldn't want to bite into a beautiful bite-size morsel and get a hard royal icing flower.