Preheat your oven to 320°F. Line the bottom of a 9-inch round springform pan with parchment paper and grease the sides lightly with butter. Wrap the outside of the pan tightly with two layers of aluminum foil to prepare for a water bath. Bring a kettle of water to a boil.
In a saucepan over low heat, combine the cream cheese, unsalted butter, and whole milk. Stir gently until everything is melted and smooth. Remove from heat and let cool for 5 minutes. Sift in the gluten free 1-to-1 baking flour and cornstarch, whisking until no lumps remain.
Whisk the egg yolks into the cream cheese mixture one at a time, then stir in the lemon juice and vanilla extract until smooth. Set aside.
In a large, clean bowl using an electric mixer, beat the egg whites and cream of tartar on medium speed until foamy. Gradually add the granulated sugar and pinch of salt, then increase to medium-high and beat until firm, glossy peaks form that hold their shape but are not dry.
Gently fold one-third of the meringue into the cream cheese mixture to lighten it. Then fold in the remaining meringue in two additions, using slow, sweeping motions from the bottom up, until the batter is uniform with no white streaks.
Pour the batter into the prepared springform pan. Place the pan in a large roasting pan and carefully pour the boiling water into the roasting pan until it comes about 1 inch up the sides of the springform pan.
Bake in the water bath for 60-70 minutes, until the top is golden and the cake jiggles gently like set custard when lightly shaken. Turn off the oven, crack the door open slightly, and let the cheesecake cool inside the oven for 30 minutes to prevent cracking.
Remove from the water bath, take off the foil, and let the cake cool completely to room temperature. Refrigerate for at least 4 hours or overnight before removing the springform ring and serving.