01 - Preheat the oven to 325°F. Line a 9-inch springform pan with parchment paper, pressing it flat against the base.
02 - Combine the crushed graham crackers, sugar, and melted butter in a bowl until evenly moistened. Press the mixture firmly into the bottom of the prepared pan to form an even layer. Bake for 10 minutes, then set aside to cool slightly.
03 - Using an electric mixer, beat the softened cream cheese and sugar together on medium speed until completely smooth and free of lumps. Add the eggs one at a time, beating briefly after each addition. Blend in the sour cream, flour, vanilla extract, and lemon zest, mixing until just combined. Avoid overmixing.
04 - Pour the filling over the par-baked crust, smoothing the top with a spatula. Gently tap the pan on the countertop a few times to release any trapped air bubbles.
05 - Bake on the center rack for 50 to 60 minutes, until the outer edges are set and the center still has a slight jiggle when gently shaken.
06 - Turn off the oven and crack the door open slightly. Let the cheesecake rest inside the cooling oven for 1 hour to prevent cracking from rapid temperature changes.
07 - 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.
08 - Transfer the cheesecake to the refrigerator and chill for at least 6 hours, preferably overnight, until fully set.
09 - Run a thin knife around the inside edge of the pan to loosen the cheesecake, then release and remove the springform ring. Slice into portions using a sharp knife dipped in hot water for clean cuts, and serve.