I'm frosting a cake with buttercream and cutting out some circle designs with fondant...my question is once I attach the circles on to the cake it should stick...right? or do I need something else? and also the other way around, if I was covering a cake with fondant...can I use buttercream to attach the circle cut outs to the cake
As long as your buttercream is still sticky (meaning it hasn't crusted over if it is that type of buttercream) you should be able to easily stick fondant decorations to it. If it has crusted over you can use a bit more buttercream on the back of the fondant or even a touch of water or piping gel or melted chocolate to help adhere it.
For a fondant covered cake you can use the same things I noted above as glue. For me, I always have a tub of piping gel so I just use that to attach decorations.
The best way is to pipe an open circle of b’cream on the back of the fondant item using a tip #3,4,5 or 6 (depending on how strong your squeeze &/or the size of the decoration) If the cake is fondant covered just pipe with b’cream — no problem —no different than as if it were all b’cream Another option is to use a dab of melted chocolate/candy melts When attaching you will need to hold it in place for maybe 20-30 seconds giving the chocolate time to start to set up/ grab hold
Fondant becomes sticky when wet. You can lightly brush the back of your items with water to adhere them.