01 - Preheat oven to 320°F. Line bottom and sides of 8-inch round cake pan with parchment paper. Wrap pan exterior with aluminum foil to prevent water seepage.
02 - Set heatproof bowl over simmering water pot. Melt cream cheese, butter, and milk together, stirring until smooth. Remove from heat and cool slightly.
03 - Sift cake flour, cornstarch, and salt together. Add to cream cheese mixture and whisk until fully incorporated.
04 - Mix in egg yolks, lemon juice, vanilla extract, and lemon zest if using. Whisk until smooth and set aside.
05 - Beat egg whites with electric mixer on medium speed until foamy. Gradually add 5 tbsp sugar in three portions, beating until stiff glossy peaks form.
06 - Gently fold one-third of meringue into cream cheese batter to lighten. Carefully fold in remaining meringue in two additions without deflating.
07 - Pour batter into prepared pan. Tap pan gently on counter to release trapped air bubbles.
08 - Place cake pan inside larger roasting pan. Pour hot water into roasting pan until halfway up cake pan sides.
09 - Bake for 60-70 minutes until golden, set, and skewer inserted in center comes out clean.
10 - Turn off oven and leave cheesecake inside with door ajar for 15 minutes to prevent cracking.
11 - Remove from oven and cool completely in pan before unmolding. Refrigerate at least 2 hours before serving.