Toast the black sesame seeds in a dry skillet over low heat until fragrant. Let them cool slightly, then grind them into a fine powder.

In a saucepan, whisk together the ground sesame seeds, milk, and sugar. Bring the mixture to a gentle simmer over medium heat, stirring continuously.

Stir the cornstarch mixture into the simmering sesame milk, and continue to cook and stir until the pudding thickens, about 2-3 minutes.

Pour the pudding into serving dishes and let it cool to room temperature. Refrigerate until set and chilled before serving.