My favorite hot chocolate is at a chocolatier just off of Harvard Square called Burdick's. I can't afford the chocolate by the piece, so I get a small hot chocolate for dessert.
52D Brattle St
Cambridge, MA 02138-3731
+1 617 491 4340
8am-11pm Tue-Sat; 9am-9pm Sun-Mon
http://www.laburdick.com/