I make hot cocoa from cocoa powder pretty often and I've been known to experiment with its strength and what I put into it for sweetener. Chocolate that you can drink is a different experience though because the cocoa butter in the chocolate makes it so much more complex and rich.
I'd had it before in a couple of high-end restaurants but never took it seriously until a visit to Ghirardelli Square in San Francisco. They make their own drinking chocolate product which you can sample at their shop for $4.50 or buy in a can and take home. Lots of other companies make preparations for drinking chocolate, but how hard, thought I, could it really be?
A lot of room for experimentation exists and I intend to continue to experiment with reckless abandon, time permitting, but here's something that I think works pretty well:
I started with 20 grams of dark chocolate, in this case, Poulain Noir Extra (47%). I chopped the chocolate and put it in my little stainless milk-steaming pitcher then melted it under the milk-steaming head of my espresso machine. I started with the valve just cracked open so as not to put chocolate all over God's green Earth. I stirred it a little to make sure that it was totally melted and smooth then added 5cl (2fl oz.) of 2% milk and steamed and frothed the ensemble under the milk steaming head. This gives me a concoction that is roughly 1/3 chocolate, 1/3 water, and 1/3 milk. My espresso machine is old and I think it lets too much water through when steaming. I was shooting for an espresso-sized serving but I got a watered-down 2 expresso-sized servings. I usually put a dash of Cayenne on top (it's very traditional). The whole one or two servings (depending on your steamer and how greedy you are) should have about 130 Calories.
If you don't have an espresso steamer or you don't want to deal with the mess, you can do this in the microwave, but it's not as good and you're in danger of scorching the chocolate: In a coffee mug, put the chocolate and about a tablespoon of water. Microwave on high 30 seconds, stir, microwave on high another 20 to 30 seconds. Stir well until you obtain a chocolate syrup. Now add the milk, stir, and microwave another 30 seconds. Here, if you want to, you can just drink it cool, skipping the final microwaving. The Ancients usually took Chocolate cold, I recommend trying it both ways. If you want the froth effect, take a wire whisk that's small enough to fit in your coffee mug and spin it between your palms like a Mayan trying to start a fire with a stick. After about 30 seconds, your milk should be fairly frothed.
I find that, using the microwave method, I end up with a little chocolate residue of un-melted un-dissolved chocolate. Not the end of the world, but a detractor to the overall presentation.