Combine 2 tablespoons of the milk with the cornstarch in a small bowl to make a smooth slurry.
Chop the chocolate and place it in a medium bowl.
Fill a large bowl with ice and water.
Combine the remaining milk, the cream, evaporated milk, sugar, and corn syrup in a 4-quart saucepan, and bring to a rolling boil over medium-high heat. Add the cocoa, whisking until it is incorporated, and continue to boil for 4 minutes.
Remove the pan from the heat and gradually whisk in the cornstarchWhisk slurry. Bring the mixture back to a boil over medium-high heat and cook, stirring with a heatproof spatula, until it thickens, about 1 minute. Remove from the heat.
Gradually whisk the hot milk mixture into the chocolate. Add the salt and whisk until the chocolate is melted.
Pour the mixture into a 1-gallon resealable plastic freezer bag and submerge the sealed bag in the ice bath. Let stand, adding more ice as necessary, until the mixture is chilled through, about 30 minutes.
Pour the ice cream base into the ice cream maker and process until thick and creamy per the machine’s instructions.
Pack the ice cream into a resealable container, press a sheet of parchment directly against the surface of the ice cream, and seal it with an airtight lid. Stash it in the coldest part of your freezer until firm, at least 4 hours. Scoop into bowls, dollop it onto a cone, or simply grab a spoon.