Preheat oven to 350 degrees F (175 degrees C). Lightly grease a springform pan.
Mix the cream cheese and ricotta cheese together in a mixing bowl until well combined. Stir in the sugar, eggs, lemon juice, vanilla, cornstarch, flour, and butter. Add the sour cream last and stir. Pour the mixture into the prepared springform pan.
Bake in the preheated oven 1 hour; turn oven off and leave in oven 1 hour more. Allow to cool completely in refrigerator before serving.
2 (8 ounce) packages cream cheese, softened
1 (16 ounce) container ricotta cheese
1 1/2 cups white sugar
1 tablespoon lemon juice
1 teaspoon vanilla extract
3 tablespoons cornstarch
3 tablespoons flour
1/2 cup butter, melted and cooled
1 pint sour cream