Dark molasses isn’t that great a sweetener for hot cocoa. Honey works fine, though I’m not the world’s biggest fan of honey.
I have a feeling I can get the amount of sweetening pretty low and enjoy the drink, because I am relatively fond towards bitter flavors. A drink made just of cocoa powder and skim milk probably would be good-for-you stuff, if a person tolerated its flavor and took it in moderation.
BTW I saw yesterday that they make chocolate with the theobromine removed (and I imagine also the caffeine), so your doggy can enjoy chocolate without, um, dying.