Cook's Note: If you find Mexican chocolate that is already sweetened and flavored with cinnamon, like Ibarra or La Fronterra, omit the sugar and cinnamon stick in this recipe. You can also replace the whole milk with almond or soy milk.
In a medium saucepan, heat the milk with the cocoa powder, powdered sugar, cinnamon stick and salt, whisking constantly until it comes to a full boil.
Remove from the heat and add the chocolate, whisking gently until it's completely melted. Remove the cinnamon stick. If desired, blend the mixture with an immersion blender until smooth and frothy. Serve warm.