Blend together ricotta and cream cheese, and add sugar. Beat in eggs, one at a time. Add lemon juice, vanilla, cornstarch, flour and butter. Mix well. Fold in sour cream and pour into buttered 25 cm springform pan. Put cake in a cold oven. Heat to 325 degrees, and bake for one hour. Turn oven off and leave cake in for two more hours. Do not open oven door while cooling. Remove cake and finish cooling. Wrap and refrigerate. Cake is best the next day.