Place 2 tablespoons of the milk in a small bowl. Whisk in the cornstarch to make a slurry and set aside.
In a pot, add the remaining milk and bring to a boil over medium heat. Whisk in the slurry, corn syrup, sugar and salt. Return the mixture to a boil and whisk in the chocolate until completely smooth. Transfer into a bowl and let cool to room temperature. Once cooled, stir in the vanilla extract. Cover and refrigerate for at least 3 hours or preferably overnight to make sure it is completely chilled.
Whisk the base and then pour it into your ice cream maker and churn. The gelato should be the consistency of soft-serve ice cream. You don't need to churn it as long as you do ice cream - you want it to just be a custard sort of consistency, not completely frozen. Store in a chilled container in the freezer, with plastic wrap pressed onto the top.
(After the gelato has been in the freezer for a day, take it out about 10 minutes before ready to serve to let it soften up).