First, a repeat: Cafe Zing in Porter Square. I really want this hot chocolate to be awesome, but it's merely okay. I had the spicy "Aztec" hot chocolate, and while I liked the level of sweetness, I found it lacking in chocolateyness and I wanted it to be spicier, and clearer in flavor. Overall: B-
Second, a new highlight!: Blue Tierra Chocolate Cafe in South Boston. This was my first time visiting here; until Saturday, I didn't even know this place existed! The hot chocolate was great: it was rich, chocolatey, with a pleasantly full-bodied mouthfeel, and a nice balance of sweetness and complexity. This gets a solid A.
Blue Tierra is a full-on chocolate shop, so in addition to a hot chocolate, I had a caramel-filled pyramid, which I found delicious. On the other hand, the default was to serve the hot chocolate in a paper cup, even though we ordered in. They did make it in a (very lovely porcelain) teacup for me on request, but not so with eestiplika's chocolate tea, which they would only serve in a paper cup. Also, the chocolate-enrobed cinnamon marshmallow she ordered was disappointing. I'd like to go back there and try other stuff here to see if it's all so hit-or-miss, but, of course, it's a bit out of the way, so it'll probably be a while before I get back there.