Prepare pan: Wrap 9-inch springform pan tightly with two layers of heavy-duty foil. Spray inside with non-stick spray. Preheat oven to 325°F. Place roasting pan in oven.
Make crust: Mix graham cracker crumbs, sugar, and salt. Add melted butter and stir until mixture resembles wet sand.
Bake crust: Press mixture firmly into bottom of pan. Bake 10 minutes until lightly golden. Cool completely.
Beat cream cheese: Beat softened cream cheese on medium speed 3-4 minutes until completely smooth and fluffy. Scrape bowl frequently.
Add sugar: Add sugar, flour, and salt. Beat on medium-low 2 minutes until smooth and creamy.
Add sour cream: Add sour cream, vanilla, lemon juice, and lemon zest. Beat on low just until smooth, about 1 minute.
Add eggs: Add eggs and egg yolk one at a time on low speed, beating just until incorporated after each. Scrape bowl between additions.
Final mix: Stir gently with spatula to ensure everything is incorporated. Batter should be thick, smooth, and pourable.
Pour and prep: Pour filling over cooled crust. Tap pan on counter 3-4 times to release air bubbles.
Water bath: Place foil-wrapped pan in roasting pan. Pour hot water into roasting pan until halfway up sides of springform pan.
Bake: Bake 65-75 minutes until edges are set but center still jiggles like jello. Center should register 150°F on thermometer.
Cool gradually: Turn off oven and crack door open 4 inches. Cool in oven 1 hour. Remove from water bath and foil. Run knife around edge. Cool to room temperature (2-3 hours).
Optional topping: Whisk sour cream, sugar, and vanilla. Spread over cooled cheesecake. Bake at 350°F for 5 minutes. Cool completely.
Refrigerate: Cover with plastic wrap and refrigerate minimum 8 hours, preferably overnight.
Serve: Remove springform ring. Slice with hot, clean knife, wiping between cuts.