Preparing the Red Velvet Cake Bottom:

1. Preheat your oven to 350°F (175°C). Line a 9-inch springform pan and an 8-inch round cake pan with parchment paper; spray with non-stick spray.
2. In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and salt. Set aside.
3. In a large bowl, cream together the unsalted butter and granulated sugar until light and fluffy. This can take about 3-4 minutes with an electric mixer on medium speed.
4. Add the sour cream, vegetable oil, and pure vanilla extract to the butter and sugar mixture. Beat until well combined. Add the eggs one at a time, beating well after each addition. Then, add the red food coloring and mix until evenly colored.
5. Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Begin and end with the dry ingredients. Mix until just combined.
6. In a small bowl, combine the baking soda and white vinegar. Allow it to fizz, then quickly add it to the batter and mix until incorporated.
7. Divide the batter evenly between the prepared pans. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool completely in the pans on a wire rack.

Preparing the Oreo Cheesecake Filling: 8. Blend the Oreo wafers into fine crumbs. Press the crumbs firmly into the bottom of the 9-inch springform pan to form the crust. 9. In a large bowl, beat the full-fat cream cheese, Oreo filling, and granulated sugar until smooth. Add the cornstarch and beat until combined. 10. Add the sour cream and pure vanilla extract, beating until smooth. Add the eggs and egg yolk one at a time, beating well after each addition. Fold in the Oreo crumbs and chopped Oreos. 11. Pour the cheesecake mixture over the cooled red velvet cake base in the springform pan.

Baking the Cheesecake: 12. Wrap the bottom of the springform pan in aluminum foil to prevent water from leaking in. Place the pan in a larger baking dish and fill the dish with hot water until it reaches halfway up the sides of the springform pan. 13. Bake at 300°F (150°C) for about 1 hour and 30 minutes, or until the center is set and the top is slightly golden. Turn off the oven and let the cheesecake cool in the oven with the door slightly open for 1 hour. Then, refrigerate overnight.

Preparing the Ganache Topping: 14. In a microwave-safe bowl, combine the dark chocolate chips and heavy cream. Microwave in 30-second intervals, stirring between each, until smooth and glossy. 15. Pour the ganache over the chilled cheesecake, spreading it evenly over the top.

Decorating the Cheesecake: 16. Crumble the reserved red velvet cake and press the crumbs onto the sides of the cheesecake for a festive look. 17. Use the store-bought cream cheese frosting to decorate the top of the cheesecake. You can pipe it into decorative swirls or simply spread it smoothly over the top. 18. Finally, garnish the cheesecake with chopped Oreos for an extra crunch and visual appeal.