Preheat oven to 320°F (160°C). Line a 12-cup muffin tin with paper liners. Boil water for a water bath.

In a double boiler over simmering water, melt the cream cheese, butter, and milk together until smooth. Remove from heat.

Whisk the egg yolks and vanilla into the slightly cooled cream cheese mixture. Sift in the cake flour and cornstarch, whisking gently until just combined.

In a separate clean bowl, beat egg whites until frothy. Add cream of tartar. Gradually add granulated sugar while beating on medium speed until soft peaks form.

Gently fold one-third of the meringue into the cream cheese batter to lighten it. Then, fold this mixture back into the remaining meringue until no white streaks remain. Do not overmix.

Divide the batter evenly among the muffin liners. Tap the tin gently on the counter to release air bubbles.

Place the muffin tin inside a larger roasting pan. Carefully pour boiling water into the roasting pan until halfway up the sides of the muffin tin.

Bake at 320°F (160°C) for 25 minutes. Reduce temperature to 280°F (140°C) and bake for another 10-15 minutes until golden.

Turn off the oven, prop the door open slightly, and let cool inside for 15 minutes to prevent sinking. Remove from water bath and cool completely.

Dust generously with powdered sugar before serving.