Draw a ghost face on each marshmallow using the fine-tip black edible ink pen: two oval eyes and a small open-circle mouth. Alternatively, press two mini chocolate chips into each marshmallow for the eyes.
Combine the hot chocolate powder with the just-boiled milk (or water) in a large insulated carafe, stirring vigorously until the powder dissolves completely.
Pour the hot chocolate into each mug, filling about three-quarters full.
Float three ghost marshmallows on top of each cup, face-side up.
Drizzle a swirl of caramel sauce or chocolate syrup over the marshmallows if using.
Serve immediately, or keep the carafe on the lowest setting of a slow cooker so guests can pour at their own pace throughout the party.