Preheat oven to 320°F (160°C). Line the bottom and sides of an 8-inch round cake pan with parchment paper. Prepare a water bath by placing the pan in a larger baking dish.

Melt cream cheese, butter, and milk in a double boiler (or heatproof bowl over simmering water), stirring until smooth. Remove from heat and let cool slightly.

Sift in cake flour, cornstarch, and salt into the cream cheese mixture. Stir until just combined.

Add egg yolks, one at a time, mixing until smooth. Set aside.

Beat egg whites in a clean bowl until foamy. Add cream of tartar (if using) and gradually add sugar, beating until soft peaks form.

Fold egg whites gently into the cream cheese mixture in 3 parts, using a spatula, until well incorporated.

Pour batter into the prepared pan. Tap gently to release bubbles.

Bake in water bath for 60–70 minutes, or until the top is lightly golden and a skewer inserted comes out clean.

Cool in oven with the door slightly open for 15–20 minutes to prevent shrinkage.

Chill or serve at room temperature. Dust with powdered sugar if desired.