Put the chocolate into a bowl or liquid measuring cup and put a fine mesh sieve over the rim.
Add the milk, cinnamon stick and jalapenos, seeds included, to a medium pot over low heat. Bring to a simmer and let steep for 15 minutes. Stir in the vanilla. Pour the milk mixture into the sieve over the chocolate and discard the solids. Quickly stir the milk and chocolate together and pour it evenly into 4 mugs. Top with marshmallows and serve.