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, mixing until well combined.
In a separate bowl, whisk the egg yolks and then gradually add them to the cream cheese mixture, mixing until fully incorporated.
Sift the flour, cornstarch, and salt together, then fold them into the cream cheese mixture until just combined.
In another bowl, beat the egg whites with the cream of tartar until stiff peaks form. Gently fold the egg whites into the cream cheese batter in three additions, being careful not to deflate the mixture.
Spoon the batter into the prepared cupcake liners, filling each about 3/4 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.
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 cool gradually.
Remove the cupcakes from the oven and let them cool completely before serving.