Prepare the Chocolate Shortbread Crust in a greased 9-inch springform pan and set aside.
1. In the bowl of a stand mixer fitted with the paddle attachment, beat the cream cheese on medium speed for 2 to 3 minutes, until it is light and fluffy. Scrape down the sides and bottom of the stand mixer bowl to prevent lumps from forming.
2. In a separate medium mixing bowl, combine the sugar, cake flour, and salt. Stir until well combined.
3. Add the contents of the medium mixing bowl to the bowl of the stand mixer and beat on
medium speed for 1 minute. Scrape down the sides and bottom of the stand mixer bowl and
continue beating on medium speed for 2 to 3 minutes, until the mixture is smooth and creamy.
4. Reduce the mixer speed to low. Slowly add first the whole eggs and then the egg yolk to the
bowl, one at a time. After adding each, scrape down the sides and bottom of the bowl. Continue beating on low speed for 1 to 2 minutes, until the mixture is fully incorporated.
5. Add the sour cream and the vanilla to the bowl of the stand mixer and beat on low speed for
1 minute, until the mixture is smooth. The mixture should be well blended, but not overbeaten.
6. Gently fold the 1 1/2 cups chocolate chips into the batter, until evenly dispersed. Reserve the
1 tablespoon chocolate chips to evenly sprinkle over the top of the batter once it is placed in
7. Preheat the oven to 375°F. Generously grease and flour the springform pan with prepared crust. Fill the prepared springform pans with the Original Plain batter. Using an offset spatula, smooth down the batter until it is level.
8. Place the filled pans in the center of the oven, directly on the middle shelf. Bake for 15 minutes, then rotate the cakes 180 degrees to ensure even browning. (All ovens bake differently, so be alert for hot spots in your own oven.) The cakes should be starting to soufflé and should be light in color. Continue to bake for 10 minutes more, for a total of 40 minutes at 375°F.
9. Reduce the oven temperature to 250°F and leave the oven door slightly ajar. The cakes should be golden brown and lightly souffléd on the sides. Leave the cake in the oven for 10 minutes more. (This step and the two that follow allow the cakes to cool to room temperature gently, preventing cracking.) Give the cakes a gentle shake; they are done if the centers of the cakes jiggle and the surfaces of the cakes are slightly firm.
10. Turn the oven off and open the oven door wide. Leave the cake in the oven for 10 minutes more. Remove from the oven and set aside to cool to room temperature, about 1 hour.
11. Loosen the cheesecake from the springform pans by sliding an offset spatula around their inside rings. Remove the springform pan from the cake and transfer to a plate. Refrigerate for at least 8 hours or overnight. Store refrigerated for up to 5 days, or frozen for 6 months, tightly wrapped.