Preheat the oven to 320°F. Line a standard 12-cup muffin tin with paper cupcake liners or lightly grease each cup.

In a small saucepan, combine softened cream cheese, softened unsalted butter, and whole milk. Heat over low temperature, stirring continuously until smooth. Remove from heat and allow to cool slightly. Whisk in fresh lemon juice and vanilla extract.

In a separate medium-sized bowl, whisk together all-purpose flour, cornstarch, and salt. Set aside.

In a large mixing bowl, whisk egg yolks with ¼ cup granulated sugar until pale and slightly thickened. Gradually incorporate the cooled cream cheese mixture until fully blended and smooth.

Sift the reserved dry mixture into the yolk emulsion. Stir gently with a spatula until smooth and lump-free.

In a very clean, large bowl, use an electric mixer to beat the separated egg whites until soft peaks form. Gradually add the remaining ¼ cup granulated sugar while continuing to beat until stiff peaks are achieved.

Carefully fold one-third of the whipped egg whites into the cheesecake batter to lighten its consistency. Gently incorporate the remaining egg whites in two subsequent additions, using a spatula to fold until just combined.

Divide the prepared batter evenly among the cupcake cups, filling each about three-quarters full. Lightly tap the muffin tin to release any trapped air bubbles.

Bake for 20 to 25 minutes, or until the tops are lightly golden brown and a wooden skewer inserted into the center emerges clean.

Allow the cupcakes to cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. Dust with powdered sugar before serving, if desired.