Preheat oven to 320°F (160°C) and line a muffin tin with cupcake liners.
Warm cream cheese, milk, and butter over low heat until smooth, then cool until barely warm.
Whisk in egg yolks one at a time, then whisk in half the sugar and optional lemon juice. Stir in sifted flour and salt until just smooth.
Whip egg whites (with cream of tartar if using) until foamy, then gradually add remaining sugar and beat to soft peaks.
Fold whites into batter in three additions until mostly uniform. Do not overmix.
Fill liners two thirds to three quarters full. Place muffin tin in a larger pan and add hot water halfway up the sides of the muffin tin.
Bake 20 to 25 minutes until lightly golden and set with a slight jiggle. Turn oven off and cool gradually before removing.
