Preheat the oven to 320°F (160°C) and prepare a 12-cup muffin tin with cupcake liners.

In a small saucepan, melt together the softened cream cheese, milk, and butter over very low heat, stirring until smooth; remove from heat and let cool.

In a bowl, sift together the flour, cornstarch, and salt.

In a large bowl, whisk the egg yolks, half of the granulated sugar, vanilla extract, and optional lemon juice until pale and creamy; gradually mix in the cooled cream cheese mixture.

Gently fold the sifted dry ingredients into the wet mixture until just combined.

In a clean bowl, beat the egg whites until foamy; gradually add the remaining sugar and continue beating until stiff peaks form.

Fold about one-third of the whipped egg whites into the batter to lighten it, then gently fold in the remaining whites until just combined.

Divide the batter evenly among the cupcake liners, filling each about two-thirds full.

Bake for 20–30 minutes or until the tops are lightly golden and the centers are set.

Let the cupcakes cool in the tin for about 10 minutes, then transfer to a cooling rack; dust with powdered sugar before serving if desired.