Step 1: Preheat your oven to 300°F (150°C). Line a 12-hole cupcake tin with paper liners.
Step 2: In a double boiler over simmering water, melt the cream cheese, butter, and milk together until smooth. Remove from heat and let it cool slightly.
Step 3: Whisk the egg yolks and lemon juice into the cooled cream cheese mixture until well combined.
Step 4: Sift the cake flour and cornstarch into the mixture and whisk until the batter is smooth and no lumps remain.
Step 5: In a separate clean bowl, beat the egg whites with cream of tartar until foamy. Gradually add sugar and continue beating until soft, glossy peaks form.
Step 6: Gently fold one-third of the meringue into the cheese batter using a spatula. Add the remaining meringue in two more batches, folding carefully to maintain the air.
Step 7: Pour the batter into the prepared cupcake liners. Place the tin in a large roasting pan and fill the pan with hot water to reach halfway up the cupcake tin.
Step 8: Bake for 50-60 minutes, or until the tops are golden brown and a toothpick comes out clean. Turn off the oven and leave the door slightly ajar for 10 minutes before removing.
Step 9: Allow the fluffy japanese cotton cheesecake cupcakes to cool completely before serving.