Step 1: Preheat your oven to 300°F (150°C) and line a cupcake tin with paper liners. Prepare a large roasting pan that the cupcake tin can sit inside for the water bath.
Step 2: In a double boiler or a heatproof bowl over simmering water, combine the cream cheese, butter, and milk. Whisk until the mixture is smooth and completely melted.
Step 3: Remove from heat and let the mixture cool slightly. Whisk in the 4 egg yolks, vanilla extract, and a pinch of salt until well incorporated.
Step 4: Sift the cake flour and cornstarch into the wet mixture. Whisk gently until just combined and no lumps remain. Set this batter aside.
Step 5: In a separate clean bowl, beat the egg whites with cream of tartar using an electric mixer. Gradually add the sugar while beating until you achieve soft, glossy peaks.
Step 6: Gently fold one-third of the meringue into the cheese batter using a spatula to lighten it. Then, carefully fold in the remaining meringue in two batches, being careful not to deflate the air bubbles.
Step 7: Pour the batter into the prepared cupcake liners, filling them almost to the top. Place the cupcake tin inside the roasting pan and fill the pan with hot water until it reaches halfway up the sides of the tin.
Step 8: Bake for 50-60 minutes, or until the fluffy japanese cotton cheesecake cupcakes are risen and the tops are golden brown. Turn off the oven and leave the door slightly ajar for 10 minutes to prevent sudden collapsing.
Step 9: Remove from the water bath and allow to cool completely on a wire rack before serving or refrigerating.