Mix the masa harina: In a medium bowl, whisk the masa harina with warm water until smooth. Make sure there are no lumps. Set aside.
Infuse and sweeten: In a medium saucepan, combine the dark brown sugar and cinnamon stick with 1 cup of water. Bring to a boil over medium heat, stirring until the sugar dissolves. Lower the heat and simmer for 3 minutes to infuse.
Add chocolate: Stir in the cocoa powder, followed by the milk. Heat the mixture until warm but not boiling.
Thicken with masa: Slowly pour the masa harina mixture into the pot, whisking constantly to prevent lumps. Increase the heat and bring to a gentle boil, whisking until thickened, about 3 to 5 minutes.
Serve: Remove the cinnamon stick, pour into mugs, and serve warm. ¡Listo!