In a medium saucepan, whisk together the whole milk, cornstarch, and granulated sugar until the cornstarch is completely dissolved and no lumps remain.

Place the saucepan over medium heat. Cook continuously, stirring constantly with a spatula or whisk, until the mixture thickens significantly into a smooth, glossy paste.

Remove from heat and stir in the vanilla extract. Pour the thickened custard into a lightly greased rectangular container or baking dish, smoothing the top with a spatula. Allow it to cool, then refrigerate for at least 3 hours until completely firm.

Once the custard is firmly set, invert it onto a cutting board. Carefully slice the slab into equal-sized bite-sized squares or rectangles.

Set up a dredging station with three shallow bowls: one with all-purpose flour, one with the beaten eggs, and one with panko breadcrumbs. Coat each custard square in flour, dip into the egg, and roll thoroughly in the panko.

Preheat your air fryer to 199 °C (200 °C). Arrange the coated milk bites in a single layer in the air fryer basket, ensuring they do not touch. Lightly mist the tops with vegetable oil spray.

Air-fry at 199 °C (200 °C) for about 8 to 10 minutes, flipping halfway through and misting again with oil, until the coating is golden brown and crispy.