Prunes, prisms, Pastilla Nash
It seems that the term "sugar plum" can be applied to a variety of foodstuffs, but in this case the log appears to be made of good old-fashioned prunes. A Mason jar of Armagnac that's been piled high with plump, boozy prunes presently occupies a proud position on my pantry shelf, so I consider myself to be part of the pro-prune posse, as it were, but I understand why they might not want to market their product as a "prune log" in the U.S., where poor, put-upon prunes still suffer from unfortunate geriatric associations.
I'm too lazy to keep the "P" words going much longer (thankfully, right?), but if you ever see the Pastilla Nash plum/prune logs in a shop near you, you might want to pick one up and pair it with a pungent piece of cheese. Perhaps for a party.*
*An editor I used to work with called alliteration "the last refuge of the hack writer." Positively.