Prepare the Baking Pan

1. Preheat the Oven: Preheat your oven to 320°F (160°C). Line the bottom of a 7-inch round cake pan with parchment paper, and wrap the outside of the pan with aluminum foil to prevent water from seeping in during baking.
2. Prepare the Water Bath: Place the prepared cake pan inside a larger roasting pan. You’ll add hot water to the roasting pan during baking to create a steam bath, which helps the cheesecake cook evenly and prevents cracking.

Make the Cheesecake Batter

1. Melt the Butter and Cream Cheese: In a saucepan, combine the cream cheese, butter, and milk. Heat over low heat until melted and smooth, stirring frequently to prevent burning. Remove from heat and let cool slightly.
2. Add the Egg Yolks: Once the cream cheese mixture has cooled a little, whisk in the egg yolks, one at a time, until fully incorporated.
3. Add Citrus Zest and Juice: Stir in the citrus zest and juice, ensuring the mixture is well combined.
4. Sift the Dry Ingredients: In a separate bowl, sift together the flour, cornstarch, and salt. Gradually add the dry ingredients to the wet mixture, whisking constantly to avoid lumps. Set aside.

Whip the Egg Whites

1. Whip the Egg Whites: In a clean, dry bowl, beat the egg whites with a pinch of cream of tartar until soft peaks form. Gradually add the sugar, one tablespoon at a time, and continue beating until stiff peaks form.
2. Fold the Egg Whites: Gently fold the whipped egg whites into the cream cheese mixture. Use a spatula to carefully incorporate the egg whites without deflating them. This will give the cheesecake its light, airy texture.

Bake the Cheesecake

1. Pour the Batter into the Pan: Pour the cheesecake batter into the prepared pan, smoothing the top with a spatula.
2. Bake in the Water Bath: Carefully pour hot water into the roasting pan, filling it halfway up the sides of the cheesecake pan. Bake in the preheated oven for 60 minutes. Then, lower the oven temperature to 280°F (140°C) and bake for an additional 45 minutes.
3. Check for Doneness: After the total baking time, the cheesecake should be golden brown on top and firm to the touch but slightly wobbly in the center. If necessary, bake for an additional 10-15 minutes, keeping an eye on it to prevent overbaking.
4. Cool in the Oven: Once done, turn off the oven and leave the cheesecake in the oven for 1 hour with the door slightly ajar. This gentle cooling process helps prevent cracks.

Chill the Cheesecake

1. Remove from the Oven: After the cheesecake has cooled in the oven, remove it from the water bath. Let it cool to room temperature, then refrigerate for at least 4 hours, or preferably overnight. This chilling time allows the flavors to set and the texture to firm up.

Serve and Enjoy

1. Release the Cheesecake: Before serving, run a knife around the edge of the cheesecake to release it from the pan. Carefully remove the cheesecake from the pan and transfer it to a serving platter.
2. Garnish: For an extra pop of citrus flavor, garnish with a few more zest curls or thin slices of citrus fruit on top.
3. Serve Chilled: Slice the cheesecake into wedges and serve chilled. The creamy, airy texture and tangy citrus notes will make each bite a delightful experience.