Heat the oven to 350°F (175°C). Lightly grease a 9-inch springform pan and wrap its outside tightly with two layers of heavy-duty aluminum foil if using a water bath.

Stir together the graham cracker crumbs, melted butter, sugar, and salt. Press the mixture firmly into the bottom of the pan, bake for 8–10 minutes, and cool slightly.

Reduce the oven temperature to 325°F (165°C). Beat the softened cream cheese on low to medium-low speed until smooth, then add the sugar and mix just until incorporated.

Mix in the sour cream, scraped vanilla-bean seeds, vanilla extract, lemon juice, salt, and optional flour or cornstarch. Add the eggs and yolk one at a time on low speed, mixing only until each disappears.

Pour the filling over the crust and smooth the surface. Place the springform pan in a larger roasting pan and add enough boiling water to reach about halfway up the springform pan.

Bake for approximately 70–90 minutes, until the outer edge is set and the center still has a gentle wobble. Turn off the oven, prop the door open slightly, and leave the cheesecake inside for 30–60 minutes.

Remove the cheesecake from the water bath and let it cool to room temperature. Refrigerate it uncovered or loosely covered for at least 6 hours, preferably overnight, before removing the pan sides and slicing.

For the optional topping, stir together the sour cream, sugar, and vanilla, then spread it over the cooled cheesecake. Bake at 325°F (165°C) for about 10 minutes, or serve the topping chilled without baking.