Preheat the oven to 320°F (160°C). Line a muffin tin with cupcake liners.

In a large mixing bowl, beat the cream cheese, sugar, and butter together until smooth and creamy.

Add the milk and vanilla extract to the cream cheese mixture, and mix until well combined.

In a separate bowl, whisk the egg yolks and then add them to the cream cheese mixture, mixing until fully incorporated.

Sift the flour, cornstarch, and salt into the cream cheese mixture, and gently fold until just combined.

In another bowl, beat the egg whites with the cream of tartar until stiff peaks form.

Carefully fold the beaten egg whites into the cream cheese batter in three additions, being gentle to maintain the airiness.

Spoon the batter into the prepared cupcake liners, filling each about 2/3 full.

Place the muffin tin in a larger baking pan and fill the pan with hot water until it reaches halfway up the sides of the muffin tin (water bath).

Bake for 25-30 minutes, or until the tops are lightly golden and a toothpick inserted in the center comes out clean.

Turn off the oven and leave the cupcakes inside for an additional 10 minutes to prevent collapsing.

Remove from the oven and let cool completely before serving.