Prepare pan: Wrap 9-inch springform pan tightly with foil. Spray inside with non-stick spray. Preheat oven to 325°F. Place roasting pan with water in oven for water bath.
Make crust: Mix graham cracker crumbs, sugar, salt, and melted butter. Press firmly into bottom of pan. Bake 10 minutes. Cool.
Beat cream cheese: Beat softened cream cheese on medium speed 2-3 minutes until smooth and fluffy. Scrape bowl frequently.
Add sugar: Add sugar and cornstarch. Beat on medium-low 2 minutes until combined. Add sour cream and beat until incorporated.
Add eggs: Add eggs one at a time on low speed, beating just until incorporated after each. Scrape bowl between additions.
Add lime: Mix in key lime juice, zest, vanilla, and salt on low speed just until combined. Add food coloring if desired. Don't overmix.
Bake: Pour filling over crust. Tap pan to release air bubbles. Place in water bath. Bake 55-65 minutes until edges are set but center jiggles slightly.
Cool gradually: Turn off oven and crack door open 4 inches. Let cool in oven 1 hour. Remove from water bath and foil. Run knife around edge. Cool to room temperature (2 hours), then refrigerate 6 hours or overnight.
Make whipped cream: Beat cold heavy cream, powdered sugar, and vanilla until stiff peaks form, 3-4 minutes.
Serve: Remove springform ring. Top with whipped cream. Garnish with lime slices and zest. Slice with hot, clean knife.