1. In medium saucepan, stir cocoa with sugar and salt. Stir in milk, 2 cinnamon sticks, ancho chile, and bittersweet chocolate.
2. Heat over medium heat until, stirring constantly, until chocolate is melted and mixture is hot. Gently whisk to completely homogenize mixture. Simmer over low heat, whisking occasionally, until fragrant, about 10 minutes. Carefully drain and discard chile and cinnamon sticks
3. Return to pot and add mezcal.
4. Heat through. Using a hand blender, a milk frother, or a whisk, beat chocolate until foamy, then divide into four glasses.
5. Garnish with chile powder, cocoa nib (or chocolate shavings), and a cinnamon stick.
6. Serve immediately