Preheat your oven to 320 degrees Fahrenheit and line a muffin tin with paper liners.
Place a roasting pan with one inch of hot water on the bottom oven rack to create steam.
Melt the cream cheese, butter, and milk together in a heatproof bowl over simmering water until smooth.
Whisk the egg yolks and vanilla extract into the warm cream cheese mixture.
Sift the cake flour and cornstarch into the bowl and whisk until just combined.
Beat the egg whites and cream of tartar in a clean bowl until frothy.
Add the granulated sugar to the whites gradually, beating until stiff, glossy peaks form.
Fold the egg whites into the cream cheese batter gently in three additions using a spatula.
Divide the batter into the muffin cups and place the tin on the middle rack above the water bath.
Bake for 25 minutes, then reduce the heat to 280 degrees and bake for another 15 minutes.
Cool the cupcakes inside the turned-off oven with the door ajar for 15 minutes to prevent collapsing.
