Preheat the oven to 325°F and line a muffin pan with cupcake liners. Prepare a shallow pan of hot water and place it in the oven to create a gentle water bath.
In a small saucepan over low heat, gently melt the cream cheese, butter, and milk together, stirring constantly until completely smooth. Remove from heat and allow to cool slightly.
Transfer the cream cheese mixture to a mixing bowl. Whisk in the egg yolks one at a time until the batter is smooth and glossy.
Sift the cake flour and cornstarch together and gently fold them into the batter along with the vanilla extract.
In a clean bowl, whip the egg whites until foamy. Add cream of tartar and gradually add sugar while continuing to whip.
Whip the meringue until soft, glossy peaks form. The mixture should be smooth and flexible, not stiff.
Fold one third of the meringue into the cheesecake batter to lighten it, then gently fold in the remaining meringue in two additions.
Spoon the batter evenly into cupcake liners, filling each about three quarters full.
Place the muffin pan in the oven above the water bath and bake for 18 to 22 minutes until set with a slight jiggle in the center.
Turn off the oven, crack the door open slightly, and allow the cupcakes to cool in the oven for 10 minutes.
Remove the cupcakes and let them cool completely at room temperature before serving.