I first got the idea for bitter hot chocolate from Toi, Moi et Fumée,** who serve it that way, with a sugar bowl so you can sweeten it to your taste. I found that I liked my hot chocolate very bitter indeed, with just barely enough sugar to take the edge off the cocoa.
Here's how you make it so that the cocoa dissolves properly. It's usually not worth the effort if you're only making it for one, because it's a pain to deal with one mug's worth of milk on the stove, but if there's two or more of you, it's perfect.
cocoa powder (NOT hot chocolate powder)
pot large enough to hold all the milk
tea strainer like this (plastic is also fine)
funnel or ladle
mugs, spoons, and so forth for serving.
Measure the milk by filling the mug you'll be drinking out of until it's just slightly fuller than you would want, and then pouring it into the pot. Do that as many times as there are people.
Put the pot on the stove at medium heat and let it warm a bit (it shouldn't be close to boiling). Add the cocoa, about 2.5-3tbsp/person, as follows: over the pot, dump about 1.5tbsp cocoa into the tea strainer, then tap the tea strainer on the edge of the pot to sift the cocoa into the milk, while stirring constantly with the wooden spoon. Make sure that the first strainer's worth of cocoa is fully dissolved before you add the next one. Keep this up until all the cocoa is dissolved in the milk. At this point the milk should be quite warm. If it isn't, keep it on medium heat, stirring constantly, until it is. You may have to turn the heat down at some point during the dissolving process to keep the milk from boiling, which you do not want. Also, if you really stir constantly it will prevent the milk from acquiring a skin.
Dish up the hot cocoa using the funnel or ladle and serve with a sugar bowl and spoons. I like to add just under a teaspoon of sugar to my mug, but ymmv.
I'm very proud of this recipe, as I invented it my very own self. It's kind of a no-brainer, but my biggest problem with hot cocoa was always getting the cocoa properly dissolved, and I can now declare that no longer a problem.
*Which has no business being spelled like that, but WordReference.com swears up and down that there's no accent on that e. Any francophones care to weigh in?
**I guess we have to rename it now that Quebec has gone smoke-free indoors