Set a metal strainer over a large bowl and set it aside for later. 

Place the milk, sugar, cocoa powder, corn syrup (if using), and salt in a medium saucepan. Heat over medium heat until the mixture is hot and comes to a gentle simmer.

Remove from the heat and add 4 ounces of chopped chocolate, stirring until the chocolate melts. 

Whisk the egg yolks in a medium size bowl. 

Pour about ½ cup of the warm milk mixture into the egg yolks while constantly whisking. This step will temper or gently warm the eggs so they don’t turn into scrambled eggs. 

Now pour the tempered egg mixture into the hot milk, whisking constantly. Cook over medium-low heat, stirring constantly for 5 to 8 minutes until it thickens slightly. If you have an instant-read thermometer, it should read 180°F. If you don't have an instant-read thermometer, look for the mixture to be steamy hot (but not boiling) and thick enough to leave a trail when you run your finger across a spatula.  

Pour the mixture through a metal strainer and discard any lumps. 

Stir in the heavy cream and vanilla extract. Cover the mixture with plastic wrap and refrigerate for 4 hours or until chilled. 

Pour the mixture into an ice cream maker and follow the manufacturer's directions for churning ice cream. 

When the ice cream is the consistency of soft serve, stir in the chopped chocolate. 

Zap the marshmallow fluff in the microwave in 10-second increments until it is thin enough to pour. It should be about the consistency of honey. 

Pour half of the chocolate ice cream into a loaf pan. Spoon half of the marshmallow fluff and half of the caramel syrup over the ice cream, then swirl it around with a knife. 

Top with the rest of the ice cream, then swirl in the rest of the marshmallow fluff and caramel syrup. 

Press plastic wrap over the top of the ice cream and place it in the freezer for 6 to 8 hours.