Preheat your oven to 320°F (160°C). Grease and line the bottom of a round 8-inch (20 cm) cake pan with parchment paper. Prepare a large baking dish or tray that can hold the cake pan and will serve as the water bath.

In a small saucepan over low heat, melt the butter and milk together. Stir constantly until the butter is fully melted and the mixture is smooth. Remove from the heat and let it cool slightly.

Gradually add the melted butter and milk mixture to the cream cheese, stirring continuously until well combined and silky. Then mix in the vanilla extract and egg yolks one at a time, beating well after each addition.

In a separate bowl, beat the softened cream cheese until smooth and lump-free.

In a medium bowl, sift together the cake flour, cornstarch, and salt. Gradually add the dry ingredients to the cream cheese mixture, stirring gently until combined.

In a separate, clean bowl, whisk the egg whites until they become frothy. Add the cream of tartar (if using) and continue whisking until soft peaks form. Gradually add the granulated sugar while continuing to beat the egg whites until they form stiff, glossy peaks.

Gently fold the beaten egg whites into the cream cheese mixture in three additions. Use a spatula to fold carefully so as not to deflate the batter.

Pour the cheesecake batter into the prepared cake pan and smooth the top. Place the cake pan into the larger baking dish or tray. Pour hot water into the larger dish to create a water bath, about 1 inch deep.

Carefully transfer the whole setup to the oven and bake at 320°F (160°C) for 50-60 minutes, or until the top of the cheesecake is golden brown and a toothpick inserted in the center comes out clean.

Turn off the oven and leave the cheesecake in the oven with the door slightly ajar for about 15 minutes. This helps to prevent it from collapsing too quickly.

Remove the cheesecake from the oven and the water bath. Let it cool completely in the pan. Once cooled, carefully remove the cheesecake from the pan and peel off the parchment paper.

For an extra elegant touch, dust the top of the cheesecake with powdered sugar before serving. Slice and enjoy the delicate, fluffy layers of your Japanese Steamed Cheesecake!