Start your campfire an hour ahead, so you’ll have a nice bed of coals. Split graham crackers in half. Split chocolate bars in half. Place chocolate on top of each cracker. Spear marshmallow with green twig and roast near coals, caramelizing to a deep brown (be careful not to burn). Enlist a friend or family member to hold two chocolate-topped crackers. Place roasted marshmallow on top of one piece of chocolate. Have your helper place the second cracker on top, chocolate side down. Hold the “sandwich” firmly and extract your roasting twig, leaving the warm marshmallow inside. Gently smoosh sandwich (be careful of oozing hot chocolate).