Sieve cottage cheese into large mixing bowl. Add softened cream cheese, beat at high speed until well blended and creamy. Still beating add sugar, then eggs. Reduce speed to low add cornstarch, lemon juice and vanilla. Beat until well blended. Add melted butter and sour cream, beat until smooth. Pour into prepared pan.
Bake in 325-degree oven about 1hour and 10 minutes or until firm around edges. Turn off oven. Let stand in oven for 2 hours. Remove to wire racks and let cool.
Chill and then remove the sides of the spring pan and serve.