Whisk together cornstarch and 1/2 cup milk until smooth.
Cook remaining 3 1/2 cups milk in a large, nonaluminum saucepan over medium heat until bubbles appear around edge of saucepan (about 4 minutes; do not boil). Whisk in chocolate, Southern Comfort, honey, vanilla extract, and salt until blended and smooth. Whisk in cornstarch mixture.
Bring milk mixture to a light boil, whisking frequently (about 4 minutes). Remove from heat. Let cool slightly. (Mixture will thicken as it cools.) Serve immediately with Marshmallow Whipped Cream, if desired.
Note: For testing purposes only, we used Ghirardelli Intense Dark Twilight Delight 72% Cacao dark chocolate bar.