Preheat the oven to 320°F and position a rack in the center of the oven.

Wrap the outside of an 8-inch round cake pan tightly with two layers of foil to prevent leaks and set it aside.

Bring a kettle of water to a simmer for the water bath.

In a heatproof bowl, combine the cream cheese, butter, and milk.

Place the bowl over a saucepan of gently simmering water and whisk until completely smooth and melted.

Remove the bowl from heat and let the mixture cool slightly.

Whisk in the egg yolks one at a time until fully incorporated.

Add the lemon juice and vanilla extract and mix until smooth.

Sift the cake flour and cornstarch directly into the bowl.

Whisk gently until the batter is smooth and free of lumps.

In a separate clean bowl, beat the egg whites on medium speed until foamy.

Add the cream of tartar and continue beating until soft peaks form.

Gradually add the granulated sugar while beating, then increase speed and beat until glossy medium peaks form.

Add one third of the egg white mixture to the batter and gently fold to lighten it.

Add the remaining egg whites in two additions, folding carefully to preserve the air.

Pour the batter into the prepared cake pan and gently tap the pan on the counter to release large air bubbles.

Place the pan into a larger roasting pan and pour hot water into the roasting pan until it reaches halfway up the sides of the cake pan.

Bake at 320°F for 25 minutes.

Reduce the oven temperature to 285°F and continue baking for 45 minutes.

Turn off the oven and leave the cake inside with the door slightly open for 15 minutes.

Remove the cake from the oven and allow it to cool completely in the pan.

Once cooled, remove the cake from the pan and transfer to a serving plate.

Dust lightly with powdered sugar if desired before slicing.