Preheat the oven to 300°. In the bowl of a standing mixer, paddle the cream cheese on low until smooth, about 2 minutes. Add the sugar and mix until it is completely incorporated, 1 to 2 minutes.
In a small bowl, gently whisk together the cornstarch and salt. Add the milk in a slow, steady stream, then whisk in the egg.
With the mixer on medium-low speed, add the egg mixture to the cream cheese, beating until the mixture is smooth, about 3 to 4 minutes. Scrape down the sides of the bowl.
Pour the cheesecake batter into an 8-inch-square baking pan lined with plastic wrap. Place the pan in the oven. Bake for 15 minutes. After 15 minutes, gently shake the pan: The outside edges should be firm and the center loosely set. If the cheesecake has not reached that consistency, allow it to cook for an additional 5 minutes.
Remove the cheesecake from the oven, set aside to cool completely, then serve.