Spray a 9-inch springform pan with non-stick cooking spray. Sprinkle graham crackers into bottom of the pan.
Force the ricotta cheese through a sieve and blend with cream cheese until creamy using an electric mix on high speed. Blend the sugar and eggs together using an electric mixer on medium speed. Reduce the speed and add the cornstarch. Add the butter and sour cream. Pour mixture into pan.
Bake at 325 degrees for 1 hour and 10 minutes until it is firm. Turn the oven off and let set in over for 2 hours with the door closed.