01 - Preheat the oven to 325°F. Line a 9-inch springform pan with parchment paper, pressing it flat against the bottom.
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 - Beat the softened cream cheese and sugar together with an electric mixer 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, spreading it evenly. Gently tap the pan on the counter a few times to release any trapped air bubbles. Bake for 50 to 60 minutes, until the edges are set and the center still has a slight jiggle.
05 - Turn off the oven, crack the oven door open, and leave the cheesecake inside for 1 hour. This slow cooling prevents cracks from forming on the surface.
06 - If using the topping, whisk together the sour cream, powdered sugar, and vanilla extract until smooth. Spread the mixture evenly over the cooled cheesecake surface.
07 - Cover the cheesecake and refrigerate for at least 6 hours, or preferably overnight, until fully set and chilled through.
08 - Run a thin knife around the edge of the pan to loosen the sides, then release the springform ring. Slice with a clean, hot knife for clean edges and serve.