Heat the oven to 325°F (165°C). Grease the springform pan. Mix gingersnap crumbs and melted butter, then press across the base.
Bake crust for 10 minutes. Set a roasting pan of hot water on the lower rack to provide a humid oven environment.
Beat cream cheese and brown sugar on low until smooth, scraping often. Mix in pumpkin puree, sour cream, vanilla, spice, and flour.
Add eggs one at a time on low speed. Stop when blended, then pour filling over the crust and smooth the surface.
Bake above the water pan for 65–80 minutes. The edge should be set while a small center area wobbles as one piece.
Turn off heat and leave in the oven with the door slightly open for 1 hour. This gentle cooling reduces sudden contraction.
Cool on a rack about 1 hour, then refrigerate at least 6 hours or overnight. Cover once cold and leave the springform ring attached.
Release the ring after running a thin knife around the edge. Cut chilled slices with a warm wiped knife and serve with optional whipped cream.
