There are a lot of pics here of cakes with round circles of varying sizes on them. (Big polka dots) What are those circles made out of? Can you put them on a buttercream cake? Can you make them out of colored wafer chocolate?
Inquiring minds want to know.
A lot of them are fondant, rolled and cut out with round cutters and yes you can put fondant decorations on a buttercream cake.
Either way works, fondant or chocolate. I have a dark chocolate cake in my photos using multi-colored candy melts for polka dots. Worked really well.