Place milk and chocolate in a 12-ounce microwave-safe mug or a 2-cup glass measuring cup.
Microwave on high until hot but not boiling, 60 to 90 seconds; stir until chocolate melted.
Combine sugar, cornstarch, and salt in a small bowl; stir into milk mixture.
Microwave on high for 30 seconds; stir. Continue microwaving until mixture thickens and just begins to boil, about 15 seconds more.
