In a pot, place the hot chocolate disc, 1 cup of water, and milk into a medium saucepan. Add the cinnamon, brown sugar, vanilla, and cayenne. Heat on medium, stirring occasionally, until chocolate is melted.
In a blender, mix the masa harina with remaining 1 cup of water. Add the hot chocolate mixture to the blender, and blend with the masa until smooth.
Return the chocolate and masa to the pan and heat on medium low, stirring occasionally until thickened. If too thick, slowly add a bit more water until it reaches your preferred consistency.