Cream together the ricotta and cream cheese. Add sugar and continue beating until smooth. Add lemon juice, vanilla, flour and cornflour, beating well after each addition. Beat in eggs, one at a time. Gradually beat in melted butter. Blend in sour cream and continue beating all ingredients until well blended.
Pour into ungreased 20x30cm pan. Bake at 160 degrees C for 1 hour. Turn off oven and leave inside for 2 hours.