Hypothetically this all began as a good idea, and it is friggin not that difficult.
I cut out a lovely oval fondant. (Yay!)
Painted with outside borded christmas red just gel paste, nothing added, it wont dry, it's tacky. So frickin fine!!!
Now, I wrote my stupid little phrase on it.
Now, I'm worried if it falls off, the tacky red will go all over the white cake and it'll look really bad. I'm about to paint it with decopauge glue. lol oh my.
Any ideas how to make sure this baby does not move an inch on my buttercream. I'm so frustrated. I'm going to bed to cuddle my baby, maybe in the morning someone will have had advice for me. PLEASE!! 
I've never tried attaching a plaque to buttercream, always to fondant, but I always use royal icing. Hopefully it will work as well on your bc. Good luck!
Royal icing sounds like a good idea. I did a plaque once and attached it to buttercream on the side of my cake and I used a mix of water and corn syrup to "glue" it on. But that's not fool proof. Mine didn't fall but gosh I would hate for you to do that and have yours fall off. I hope someone with more experience in this matter can offer some help for you.
I use piping gel to attach fondant to buttercream.
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