Preheat oven to 320°F (160°C) and prepare muffin tin with liners.
Beat cream cheese, sugar, and butter until smooth. Add milk and vanilla, mix well.
In a separate bowl, whisk egg yolks, flour, cornstarch, and salt. Gradually add to cream cheese mixture.
Whip egg whites with cream of tartar until stiff peaks form. Fold into batter gently.
Spoon batter into liners, filling two-thirds full. Place muffin tin in a larger pan with hot water.
Bake for 25-30 minutes until golden and toothpick comes out clean.
Cool in the oven with the door ajar for 10 minutes, then cool completely on a wire rack.
Chill in the refrigerator for at least 120 minutes before serving.