Heat oven to 350 degrees. Mix ricotta and sour cream in a mixing bowl. Beating slowly, add cream cheese, sugar, and butter. Increase speed to medium and add eggs, flour, cornstarch, vanilla and lemon juice. Beat on highest speed possible for 5 minutes. Pour mixture into 10" springform pan. Bake at 350 degrees for 1 hour. Turn off heat and leave in oven with door closed for 1 hour. Cool on rack and then chill overnight.