Preheat oven to 350 degrees. Grease a springform pan.
Combine the softened cream cheese and ricotta cheese in a large mixing bowl.
Add sugar, eggs, lemon juice, vanilla, corn starch, flour, and butter and mix well. Add sour cream until just mixed, do not over mix.
Pour into the springform pan. Reduce heat to 325 degrees when putting in the oven and bake for 1 1/2 hours. Do not open oven. Shut off the oven but keep inside for an additional 2 hours, also without opening the oven.
Cool on the counter for 1-2 hours, then refrigerate overnight before eating.