1-Prepare the Crust: Preheat the oven to 350°F (175°C). Crush Oreo cookies finely, including the cream filling for the crust, using a food processor or by hand. Combine cookie crumbs with melted unsalted butter and press firmly onto the bottom, and optionally up the sides, of a 9-inch springform pan. Bake the crust for about 10 minutes, then allow it to cool.

2-Make the Cheesecake Filling: In a large mixing bowl, beat softened cream cheese, butter (if using), and sugar together until smooth and creamy, scraping down the sides. Add eggs one at a time, mixing thoroughly after each addition, then add the optional egg yolk. Stir in sour cream, vanilla extract, salt, and flour (if using) until the batter is smooth. Gently fold in the chopped Oreo pieces.

3-Assemble and Bake: Pour the cheesecake filling over the prepared crust and smooth the top. For creamier texture and to prevent cracking, place the springform pan inside a larger pan and add hot water to create a water bath reaching halfway up the inner pan. Bake at 320°F (160°C) for 50-60 minutes, or initially bake at 350°F for 10 minutes and then reduce to 300°F for 50 minutes. The cheesecake should be set at the edges but slightly jiggly in the center.

4-Cool and Chill: Turn off the oven, slightly crack open the oven door, and let the cheesecake cool inside for 30 minutes to 1 hour to prevent cracking. Remove from the oven and cool to room temperature. Run a knife around the edges to loosen the cake and place it uncovered in the refrigerator for at least 6 hours or overnight to set firmly.

5-Prepare the Ganache and Decorations: Heat the heavy cream until simmering, then pour over chopped dark or semisweet chocolate. Let sit 3-5 minutes, then stir until smooth. For marbled effect, prepare white chocolate ganache by mixing white chocolate and cream similarly, and swirl over chilled cheesecake. Refrigerate ganache until firm (20-30 minutes). Whip cold heavy cream with powdered sugar and vanilla extract to soft peaks. Pipe whipped cream around the edges and garnish with halved or broken Oreo cookies.

6-Serve and Enjoy: Remove the sides of the springform pan, transfer the cheesecake to a serving platter, and serve chilled.