Where in Boston? You might try calling Cantina La Mexicana in Union Square and Crema in Harvard, as well as Voltage for ideas/offerings. Salvadoran and Guatemalan atoles are reasonably common but not exactly what you are looking for. And if you are looking at making your own consider Taza's chocolate discs. For other unique chocolate drinks, Caffe Paradiso in the North End does some Italian ones different than other cafes (with coffee) and Pomplona used to offer some chocolate drinks which were very warming in winter (and different than the usual board recommendation of Burdick).