Preheat oven to 320°F (160°C). Line and prepare an 8-inch round cake pan. Set up a water bath.
Melt cream cheese, butter, and milk in a double boiler. Let cool slightly.
Sift in flour and cornstarch. Mix until smooth.
Add egg yolks and mix well. Set aside.
In another bowl, beat egg whites with cream of tartar until foamy. Gradually add sugar and beat to stiff peaks.
Fold egg whites gently into the cream cheese mixture in batches.
Pour batter into the pan and tap to remove air bubbles.
Bake in the water bath for 60–70 minutes until set and lightly golden.
Cool in the oven with the door cracked open to prevent sinking.
Chill before serving. Dust with powdered sugar if desired.