I am making a tombstone to stand up on a cake and have taken a piece of cardboard cut it the size I need and have poured melted chocolate over it. My question is should I write on it with royal icing or buttercream? I know if I use buttercream I need to wait until right before I deliver the cake but if I use royal icing will it hold up for several days? If I use royal icing will it bleed or melt off the chocolate since I put crisco in the chocolate? I have never done this before so I am needing advice. Thanks!

Either would be fine but the RI would hold better. You could also use white chocolate to write with.

I have used both. Neither bleeds nor melts off.