Preheat oven to 320°F. Line the bottom of an 8 inch round pan with parchment paper.
Melt cream cheese, butter, and milk over a double boiler until smooth. Let it cool slightly.
Whisk in egg yolks one at a time, then fold in sifted cake flour, cornstarch, and salt.
In a separate bowl, beat egg whites with cream of tartar until foamy. Gradually add sugar and beat until stiff peaks form.
Gently fold egg whites into the batter in three additions to keep the mixture airy.
Pour batter into prepared pan. Tap gently to release air bubbles.
Bake in a water bath at 320°F for 25 minutes, then lower to 285°F and bake another 55 minutes.
Turn off oven and let cake rest inside for 15 minutes with door slightly open.
Remove from oven, cool completely, and serve.