Place cocoa in a 3-quart electric slow cooker. Gradually add 1 cup fat-free milk, stirring with a whisk until well blended. Add 4 1/2 cups fat-free milk, vanilla, and condensed milk, stirring with a whisk until well-blended. Add cinnamon stick, if desired. Cover with lid; cook on low-heat setting 4 to 8 hours. Discard cinnamon stick. Stir well with a whisk before serving. Ladle hot chocolate into mugs; top with marshmallows.