Anna -
There are many different ways to do this ... all depends on what you want to achieve. For the most part I do not like the use of starch thickeners in my chocolate drinks as it changes the texture in ways I do not like. One thing however, is that (I find) it takes time to fully hydrate the starch so you don't really want to make it up a la minute. You can make a dense mousse with the starch in advance and then add the liquid
In my town, the local chocolate shop makes their (French-style sipping chocolate) in advance and dispenses it out of air pots. It's "just" chocolate and dairy - and one of the best sipping chocolates I have had this side of the Atlantic. They use very fresh organic cream. Personally, I think this is the secret - very high quality dairy.
At Bonnat in their salon, they make up the chocolate in advance and put it back in a milk jug in the fridge. They then steam it to heat it for service. At least they did back in '98 when I visited.
You could make a water-based ganache and add the (hot) dairy of choice to the ganache and then whip it using a frother or steamer. I like this approach as it makes it possible to vary the density of the product quite easily by changing the ratio of ganache to dairy - thinner for kids, thicker for adults.
You don't have to steam the milk, you could keep it hot in a coffee urn, as they do for Cuban caf con leche. This adds a nice caramel note as the lactose in the milk gets cooked.
Another idea - line the inside of the cup with a layer of tempered chocolate (any milk, white (!), or dark) and use (homemade) chocolate syrup. The hot dairy melts the chocolate lining the cup and adds interest to the drink.
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clay - http://www.thechocolatelife.com/clay/
