Picked up a few pieces from Fugetsudo yesterday. They are about the size of a silver dollar pancake. The mochi is pillowy soft and has a nice chew to it. It has a subtle yet distinctive chocolate flavor. The surprise is when you get to the middle of the mochi where there is a liquid chocolate center. It reminds me of the dobash at King's Hawaiian Bakery.
Don't know how long the chocolate mochi has been around, but the next time I'm in J town, I'll be sure to pick some more up.