01 - Preheat the oven to 325°F. Line a 9-inch springform pan with parchment paper on the bottom and lightly grease the sides.
02 - Combine the crushed graham crackers, sugar, and melted butter in a bowl until the mixture resembles wet sand. Press firmly and evenly into the bottom of the prepared pan. Bake for 10 minutes, then remove and let cool slightly.
03 - Using an electric mixer, beat the softened cream cheese and sugar on medium speed until completely smooth and free of lumps. Add the eggs one at a time, beating briefly after each addition. Fold in the sour cream, flour, vanilla extract, and lemon zest until just combined — do not overmix.
04 - Pour the filling over the cooled crust. Gently tap the pan on the counter several times to release any trapped air bubbles. Bake on the center rack for 50 to 60 minutes, until the edges are set and the center still has a slight jiggle.
05 - Turn off the oven and crack the door open. Let the cheesecake rest inside the cooling oven for 1 hour to prevent cracking from sudden temperature changes.
06 - If using the topping, whisk together the sour cream, powdered sugar, and vanilla extract until smooth. Spread evenly over the surface of the cooled cheesecake.
07 - Cover and refrigerate for at least 6 hours, preferably overnight, until fully set and firm.
08 - Run a thin knife around the edge of the pan to loosen, then release the springform ring. Slice with a clean, hot knife for clean portions and serve.