I'd use white chocolate or make it out of pastiage.
It seems like it would be pretty easy to make the rectangle shape out of thickly rolled gumpaste. I would either use gumpaste for the monogram, or just pipe it on.
i thought it woulde be easy also, but looking at it closely, it looks as if it is attached to a base of some kind...
Are you sure its edible?
I'd use pastillage or chocolate. Any way you can think of to make a mold for it? That would make it easy to make out of chocolate.
There is definitely a base on it. That's going to be the tough part, and why I think they used a mold. Maybe get some silicone, find a blank plaque with base at a craft store, ice initials in royal and leave dry, then make a mold of the whole thing?
I'd make two slabs - one for the upright, one for the base. Let them dry and then attach the two together, then let dry again.
You could do the lettering with a clay-gun if you have one, or just by piping it on. I'd do this before attaching the two slabs together.