Combine the milk and cinnamon stick in a small saucepan over medium heat, just until small bubbles form around the edge of the pan. Remove from the heat, cover the pan, and let the cinnamon stick steep in the liquid for 30 minutes. Reheat the milk over medium heat, then add the chocolate and sea salt and whisk until the chocolate is melted. Continue whisking for 2 to 3 minutes, until slightly thickened.
Remove and discard the cinnamon stick and ladle the hot chocolate into mugs.