Preheat the oven to 325°F. Grease a 9-inch springform pan and line the bottom with parchment paper if desired. Wrap the outside of the pan tightly with two layers of heavy-duty foil to help protect it from the water bath.
In a medium bowl, mix the graham cracker crumbs, granulated sugar, melted butter, and salt until the crumbs are evenly moistened.
Press the crust mixture firmly into the bottom of the prepared springform pan. Bake for 8 to 10 minutes, or until lightly fragrant and set. Set the crust aside while you make the filling.
In a large mixing bowl, combine the granulated sugar and lemon zest. Rub them together with your fingers for about 30 seconds, or until the sugar looks slightly moist and smells fragrant. Be careful to zest only the yellow part of the lemon and avoid the bitter white pith.
Add the softened cream cheese to the lemon sugar. Beat on medium-low speed until smooth and creamy, scraping down the sides and bottom of the bowl as needed.
Add the flour and mix until combined.
Add the sour cream, strained lemon juice, vanilla extract, lemon extract if using, and salt. Mix on low speed until smooth.
Add the eggs one at a time, mixing on low speed just until each egg is incorporated. Add the egg yolk and mix just until combined. Do not overmix once the eggs are added, because too much air can cause the cheesecake to puff, sink, or crack.
Pour the filling over the crust and smooth the top. Gently tap the pan on the counter a few times to release large air bubbles.
Place the springform pan inside a large roasting pan. Carefully pour hot water into the roasting pan until it reaches about 1 inch up the sides of the springform pan.
Bake for 65 to 75 minutes, or until the edges are set and the center still has a gentle wobble when the pan is lightly shaken. For the most accurate result, the center of the cheesecake should be about 148°F to 152°F. The center should not look wet or liquid, but it should still move slightly.
Turn off the oven and crack the oven door open. Let the cheesecake cool in the oven for 1 hour.
Remove the cheesecake from the water bath and take off the foil. Run a thin knife around the edge of the cheesecake to loosen it from the pan.
Let the cheesecake cool at room temperature until no longer warm, about 1 to 2 hours. Refrigerate for at least 6 hours, preferably overnight.
Once fully chilled, spread lemon curd over the top if desired. Garnish with whipped cream and extra lemon zest.
Add thin lemon slices right before serving only. Lemon slices can release juice and make the topping watery if added too far in advance.
Slice with a sharp knife, wiping the blade clean between cuts for neat slices.