In a medium saucepan, whisk together milk, sugar, and cocoa powder over medium heat until the mixture is steaming but not boiling.

In a small bowl, mix cornstarch with a tablespoon of water to create a slurry. Add the slurry to the saucepan and whisk well.

Continue whisking constantly until the mixture starts to thicken, about 5 minutes.

Remove from heat and stir in the chocolate chips until completely melted and smooth.

Pour the pudding into serving cups and let it cool for at least 20 minutes before serving.