In a large saucepan, combine cocoa powder and granulated sugar. Whisk in 1½ cups milk until fully incorporated (mixture will be a thick paste). Gradually whisk in remaining 6½ cups milk. Add cinnamon sticks and bring to a simmer, stirring occasionally. Simmer, stirring occasionally, until you can smell the cinnamon, 8 to 10 minutes.  Add chocolate and whisk until melted. Strain into a second pot, then stir in bourbon. Serve topped with toasted marshmallows.