Mix cream cheese, condensed milk, sugar, and cornstarch until well blended. Add eggs, one at a time, and beat well. Add lemon juice, and vanilla until well blended. Add sour cream and beat for 1½ minutes.
Mix cracker crumbs, sugar, and butter. Pat into bottom and up sides of 2 (4-inch) cheesecake springform pans. Set aside.
Preheat oven to 300º F. Place a pan of hot water on the bottom rack of your oven.