Preheat your oven to 320°F (160°C) and line a muffin tin with cupcake liners.
In a small saucepan, combine the cream cheese, butter, and milk over low heat. Stir until the mixture is smooth and fully combined. Remove from heat and let it cool slightly.
In a mixing bowl, whisk the egg yolks with 1/4 cup of granulated sugar until the mixture is pale and creamy. Stir in the vanilla extract, then mix in the warm cream cheese mixture until fully incorporated.
Sift together the cake flour and cornstarch, then gently fold it into the cream cheese mixture until no dry flour remains.
In a separate clean bowl, beat the egg whites with cream of tartar using an electric mixer until soft peaks form. Gradually add the remaining 1/4 cup of sugar and continue beating until glossy stiff peaks form.
Gently fold the egg whites into the cream cheese mixture in three additions, being careful not to deflate the mixture.
Spoon the batter into the prepared cupcake liners, filling each about two-thirds full.
Bake in the preheated oven for 15-20 minutes, or until the cupcakes are puffed and lightly golden on top. A toothpick inserted into the center should come out clean.
Turn off the oven and crack the door open slightly, allowing the cupcakes to cool gradually in the oven for about 10 minutes.
Remove the cupcakes from the oven and let them cool completely on a wire rack before serving.