01 - Preheat oven to 325°F. Line a 9-inch springform pan with parchment paper, pressing it firmly into the bottom and up the sides.
02 - Combine 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 softened cream cheese and sugar on medium speed until completely smooth and free of lumps. Add eggs one at a time, beating well after each addition. Blend in sour cream, flour, vanilla extract, and lemon zest until just combined — do not overmix.
04 - Pour the filling over the partially cooled crust. Gently tap the pan on the counter several times to release 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, crack the door open, and allow the cheesecake to cool gradually inside for 1 hour. This prevents cracks from forming on the surface.
06 - Whisk together sour cream, powdered sugar, and vanilla extract until smooth. Spread evenly over the cooled cheesecake surface if using the topping layer.
07 - Transfer the cheesecake to the refrigerator and chill 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 the sides, then release the springform ring. Slice with a clean, warm knife and serve.