Prepare the Crust Preheat the oven and combine the fine graham cracker crumbs with the melted unsalted butter in a medium bowl. Mix until the crumbs are evenly moistened and resemble wet sand. Press the mixture firmly into an even layer at the bottom of a 9-inch springform pan. Place the pan in the refrigerator to chill while preparing the filling.

Create the Filling In a large mixing bowl, beat the softened cream cheese, granulated sugar, and cornstarch with an electric mixer on medium speed until the mixture is completely smooth. Add the eggs and the extra egg yolk, continuing to mix until fully incorporated. Stir in the sour cream, lemon juice, and vanilla extract, mixing just until combined to avoid trapping too much air.

Bake in a Steam Bath Pour the cream cheese batter over the chilled crust and smooth the top with a spatula. Place the springform pan inside a larger roasting pan and fill the outer pan with an inch of hot water. Bake until the edges are set and the center has a slight, gentle jiggle.

Cool and Set Turn off the oven and leave the cheesecake inside with the door slightly cracked for one hour to cool slowly. Remove from the oven and let it reach room temperature before transferring it to the refrigerator. Chill for several hours or overnight to allow the texture to set perfectly before slicing.