Astoundingly easy, frothy hot chocolate with a simple technique; it's all in how you heat the milk. I use a dutch-process cocoa (Droste), but you can use any unsweetened cocoa you have on hand. —Rebecca Zicarelli
Fill your mug (or mugs) 1" below the brim and and add this to a saucepan. Place on medium-low heat, do not stir.
While the milk heats, add a splash of mile (a tablespoon or two) to each mug. Add a heaping tablespoon of cocoa and rounded tablespoon of sugar to each mug, and stir to form a smooth paste. Do not stir the mile while you're doing this, just let it heat on medium low.
Let the milk continue to heat, without stirring, until it begins to steam and a skin forms on the top, it's now time to be vigilant in watching the milk as it heats. As the milk approaches the boiling point, small bubbles will form around the edge of the pan, with more and more forming moving toward the center. When you can just begin to see the bubbles forming in the center of the pan (the milk will not yet be boiling), remove it from the heat. Tip the pan gently, the skin will flow to the top edge some, and then gently pour the frothy milk from under the skin into the mugs. Stir vigorously and, if desired, tip with marshmallows, whipped cream, or a splash of your favorite liquor.