Place cream cheese and ricotta in mixer, beat until smooth and creamy.
Add sugar in 3 additions over 1 minute, scraping bowl as necessary.
Beat 30 seconds longer.
Add eggs, one at a time, at 30 second intervals.
Mix well after each addition.
Blend in flour, cornstarch, lemon juice and vanilla.
Add butter and sour cream.
Beat for 30 seconds, scrape bowl and beat 10 seconds longer.
Wrap springform in two layers of aluminum foil, pour batter into pan, put springform in a larger pan such as a roasting pan and add hot water to larger pan until it comes halfway up the sides of the springform.
Bake until center still jiggles slightly, 1 to 1 1/2 hours.
Remove from oven, remove foil, cool completely on rack and refrigerate overnight.
To remove springform, heat sides with propane torch and remove sides.
Turn cake upside down in the palm of one hand, heat bottom with torch and remove bottom of springform.