Step 1: Preheat your oven to 325°F (160°C) and prepare a 9-inch springform pan. Grease the bottom and sides, then line the bottom with parchment paper. Wrap the outside of the springform pan tightly with several layers of heavy-duty aluminum foil to prevent water from seeping in during the water bath.
Step 2: In a large heatproof bowl set over a saucepan of simmering water (bain-marie), combine the room temperature cream cheese, unsalted butter, and whole milk. Stir continuously with a whisk until the mixture is smooth and melted. Remove from heat.
Step 3: Whisk in the Greek yogurt until fully incorporated and the mixture is smooth. Add the egg yolks one at a time, whisking well after each addition until just combined. Stir in 1/2 cup (100g) of the granulated sugar, the cornstarch, and a pinch of salt until no lumps remain and the batter is silky smooth.
Step 4: In a separate clean, dry bowl, beat the egg whites with an electric mixer on medium-high speed. Once foamy, add the cream of tartar (if using), then gradually add the remaining 1/4 cup (50g) of granulated sugar, continuing to beat until medium-soft peaks form. The whites should be glossy and hold their shape but still be slightly soft at the tip when the whisk is lifted.
Step 5: Gently fold about one-third of the whipped egg whites into the cream cheese batter using a spatula to lighten the mixture. Once mostly combined, carefully fold in the remaining egg whites in two additions. Be gentle to avoid deflating the air you’ve incorporated.
Step 6: Pour the batter into your prepared springform pan. Place the springform pan into a larger roasting pan. Carefully pour hot water into the roasting pan until it reaches about halfway up the sides of the springform pan.
Step 7: Bake for 60-70 minutes, or until the top is golden brown and the cheesecake is set but still slightly wobbly in the center. The baking time may vary slightly depending on your oven. Turn off the oven, crack the door open slightly (you can use a wooden spoon), and leave the cheesecake in the oven for another hour to cool gradually.
Step 8: Remove the cheesecake from the oven and water bath. Carefully unwrap the foil and let it cool completely on a wire rack before refrigerating for at least 4 hours, or preferably overnight. Chilling is vital for the cheesecake to firm up and for the flavors to meld beautifully. Once thoroughly chilled, gently run a thin knife around the edge of the pan before releasing the springform sides. Serve chilled and enjoy!