Coat a 10" springform pan with cooking spray and line the sides with a double layer of 6"-wide (high) waxed paper.
In a large bowl, with an electric mixer on medium speed, blend the ricotta cheese, cottage cheese, 1/2 cup of the sugar substitute, and sugar until smooth. Beat in the cornstarch, lemon juice, flour, eggs, and 2 teaspoons of the vanilla extract. Pour the batter into the prepared pan.
Bake for 1 hour and 10 minutes, or until the top of the cake is brown.
Turn the oven off and allow the cake to remain in the oven for 1 hour longer.
In a medium bowl, combine the sour cream, the remaining 1 tablespoon sugar substitute, and the remaining 1 teaspoon vanilla extract. Spread over the cheesecake and return to the oven for 10 minutes, or until the topping is set. Refrigerate for 8 hours or overnight before slicing.