I made a royal icing plague for the top of the cake should I wait to put it on top of buttercream. I'm afraid the buttercream will melt it.
How long before the cake will be served? RI doesn't melt instantaneously, but it will begin to discolor and soften over time. If you really don't want to wait, you can put some parchment (or waxed paper), cut to the exact size/shape of the plaque, between the cake and the plaque.