Preheat oven to 325°F (163°C). Line bottom of 9-inch springform pan with parchment paper and cut a strip to line the inside walls, extending 2 inches above the rim. Lightly grease the pan, press parchment in place, wrap outside tightly with 2-3 layers of foil, and fill a large roasting pan with 1 inch of hot water set on the lowest oven rack.
Combine softened cream cheese, butter, and ¼ cup sugar in a medium saucepan over barely simmering water. Stir constantly over low heat until completely smooth and lump-free. Remove from heat, pour in whole milk and stir until smooth, then add egg yolks one at a time, stirring well after each. Stir in lemon juice and zest.
Sift together cake flour, cornstarch, and salt directly into the cream cheese mixture. Whisk gently until no dry streaks remain and batter is smooth. Set aside at room temperature while preparing meringue.
Place 6 egg whites and cream of tartar (if using) in a clean, dry mixing bowl. Beat on medium speed until foamy. Gradually add remaining ½ cup sugar, one tablespoon at a time, beating on medium-high until stiff peaks form. Peaks should stand straight up without curling over.
Scoop one-third of the meringue into the cream cheese batter and gently fold using a rubber spatula until combined. Add remaining meringue in two additions, folding with a gentle motion (down the center, along the bottom, and up and over) just until no white streaks remain. Batter should be smooth, creamy, and slightly airy.
Pour batter gently into prepared springform pan and tap lightly on the counter to release air bubbles. Smooth the top with a spatula if needed. Carefully place foil-wrapped pan into the water bath in the oven.
Bake at 325°F (163°C) for 25 minutes, then reduce temperature to 300°F (150°C) and continue baking for another 50-55 minutes. Do not open the oven during the first 25 minutes. Cake is done when top is deep golden brown and center still has a very slight jiggle when shaken.
Turn the oven off but leave the cake inside with the door slightly ajar for 30 minutes. This gradual cooling prevents cracking and collapsing.
Remove springform pan from water bath and cool completely on a wire rack at room temperature for 1-2 hours. Once cooled, run a thin knife around the inside edge before releasing the springform latch. Carefully peel away parchment paper.
Transfer cheesecake to a serving plate and refrigerate for at least 4-6 hours, or overnight. This firms the texture and develops the flavors fully. Dust with powdered sugar just before serving.