In a large bowl, beat cream cheese with a mixer or food processor until completely smooth, scraping down sides of bowl occasionally. Beat in butter until mixture is smooth and creamy. Beat in sour cream. Add eggs, one at a time, scraping bowl and beating well after each addition. Add sugar, cornstarch, vanilla, lemon juice, and zest and beat until mixture is smooth. Butter sides and bottom of a 10" springform pan and completely wrap outside of pan with a double layer of heave-duty aluminum foil to make springform watertight. Pour batter into pan, place in a baking pan that is wider, but not deeper, than the springform. Add enough boiling water to reach halfway up the side of the springform. Bake in a pre-heated 300 degree F oven until top is lightly browned and a knife inserted in center comes out clean, between 2 or 2 and 1/2 hours. Remove springform pan from water bath and unwrap aluminum foil. Place pan on a rack to cool completely. Cover pan with plastic wrap and refrigerate overnight. To unmold, place springform in a pan of hot water for 10 seconds. Run a thin knife around sides of cake, release sides of the springform. Serve plain or with fresh strawberries, blueberries, or whipped cream. Alternately, dust top with cocoa or pour chocolate sauce over top. Makes 12 servings.