Combine 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 without splattering for 5 minutes. Pour mixture into a 10 inch spring-form pan. Bake in a preheated 350 degree oven for 1 hour. Turn heat off and leave in oven for 1 hour with door closed. Cool on rack.