Preheat oven to 350°F.

Mix ricotta and cream cheese until fluffy. Add sour cream and blend. Add 1 egg at a time, mixing well after each one. Add vanilla, flour, cornstarch, sugar, and butter and mix well. Add any extra fillings desired (such as chocolate chips, berries, etc).

Add graham cracker crust to bottom of buttered spring pan (follow instructions on box).

Pour mixture into buttered spring pan lined with a graham cracker crust. Bake for 1 hour or until firm. Turn off gas and leave cake in oven for 1 hour. Cover cake with foil and refrigerate overnight before serving.