Preheat oven to 325 °F (160 °C). Line a 12-cup muffin tin with paper liners and set aside. Also prepare a water bath: fill a baking pan with about 1 inch of hot water and place it ready in the oven.

In a heatproof bowl set over simmering water, combine cream cheese, butter, and milk. Stir until smooth and fully melted. Remove from heat and let cool slightly.

Whisk in egg yolks until smooth. Sift in cake flour, cornstarch, and salt, and whisk until just combined.

In a separate clean bowl, add egg whites and cream of tartar. Beat on medium speed until foamy. Gradually add sugar and continue beating until stiff, glossy peaks form.

Gently fold one-third of the meringue into the cream cheese mixture to lighten it. Then fold in the remaining meringue in two additions, being careful not to deflate the batter.

Spoon batter evenly into liners, filling about ¾ full. Tap the pan lightly on the counter to release large air bubbles.

Place the muffin tin into the prepared water bath in the oven. Bake for 25–30 minutes, or until tops are golden and a skewer inserted comes out clean.

Turn off oven and crack oven door slightly. Let cheesecakes rest in the oven for 10 minutes to prevent collapsing, then remove to a cooling rack.

Cool completely or serve warm, dusted with powdered sugar if desired.