Preheat your oven to 390°F and lightly grease a 8” x 3” round cake pan with butter. Line the bottom with parchment paper.
Pour boiling water into a large bowl or pan, then sit a smaller mixing bowl inside. Add the cream cheese to the smaller bowl and let soften while you separate your eggs. When the cream cheese is soft, whisk until smooth and creamy, then stir in butter and milk until smooth and incorporated. Whisk in the yolks until smooth, then whisk in the sugar. Remove the bowl from the hot water bath and sift in the cake flour and cornstarch. Whisk until smooth.
Whip the egg whites with the cream of tartar until frothy and pale, adding sugar bit at a time until the whites are whipped into a glossy thick meringue that holds a soft peak. Be careful not to over whip.
Take 1/3 of the whipped egg whites and whisk it into the bowl with the yolks until completely incorporated. Add half of the remaining whites and whisk into the yolk batter, being careful not to deflate.
Transfer the egg yolk mixture to the remaining egg whites, whisk and then use a spatula to fold together. Pour the batter into the prepared cake pan – it should come up almost to the top of the pan, about 1/4 inch from the top. Tap the pan against the counter to release any air bubbles.
Place a kitchen towel into a large deep baking dish and place the cake pan on top (this ensures that the cheesecake doesn’t have any contact with direct heat), then put everything into the preheated oven and carefully pour hot water about 1 inch high.
