Preheat your oven to 350°F (176°C). Grease the sides of a 9-inch (23cm) springform pan and line the bottom with parchment paper. In a medium bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt until evenly combined. In a separate bowl, whisk the milk, vegetable oil, vanilla extract, and egg white until smooth. Gradually add the dry ingredients to the wet mixture, whisking until just combined. Stir in the water—the batter will be thin. Pour the batter into the prepared pan and bake for 12-15 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Remove from oven, let cool completely on a wire rack, then clean the pan for reuse.

While the cake cools, prepare the cheesecake layer. Place the chocolate chips in a medium bowl. Heat the heavy cream until it just starts to boil, then pour it over the chocolate chips. Let sit for 3-4 minutes, then whisk until smooth. Set aside. In a large mixing bowl, beat the cream cheese, sugar, and flour on low speed until smooth and fully combined—avoid over-beating to prevent cracks. Scrape down the bowl. Add the sour cream and vanilla, mixing on low until incorporated. Add eggs one at a time, mixing slowly after each addition and scraping the bowl as needed.

Reduce the oven temperature to 300°F (148°C). Re-line the bottom of the springform pan with fresh parchment paper and spray the sides with non-stick spray to prevent sticking. Wrap the outside of the pan with aluminum foil to prepare for a water bath and avoid leaks.

Place the cooled chocolate cake back into the springform pan. Pour the melted chocolate mixture over the cake and spread evenly. Pour about one-third of the cheesecake filling over the chocolate layer and spread smoothly. Arrange 10-12 whole Oreos evenly over this layer. Add another third of the cheesecake filling on top, spreading evenly, and place another 7-8 Oreos, slightly offset from the first layer. Spread the remaining cheesecake filling on top and finish with 10-12 Oreos arranged on the surface.

Place the springform pan inside a larger roasting pan. Pour warm water into the outer pan until it reaches halfway up the sides of the springform pan—but does not rise above the foil edge.

Bake for 45 minutes. The cheesecake should be set around the edges but slightly jiggly in the center. Turn off the oven and leave the door closed for 20 minutes to let the cheesecake continue cooking gently. Then, crack the oven door open and let it cool for 30 minutes. This slow cooling helps prevent cracking.

Remove the cheesecake from the oven and water bath. Keep it in the springform pan and refrigerate for 5-6 hours or overnight until fully set.

Melt the white chocolate and 2 tablespoons (30ml) heavy cream together in a microwave-safe bowl in 10-second increments until smooth. Allow to cool to room temperature. In a large bowl fitted with a whisk attachment, whip the remaining 1 cup (240ml) heavy cream, powdered sugar, and vanilla extract until stiff peaks form. Gently fold about one-third of the whipped cream into the melted white chocolate until combined. Fold in the remaining whipped cream, then fold in the chopped Oreos.

Spread the Oreo mousse evenly over the chilled cheesecake. Return to the refrigerator for 2-3 hours, or until the mousse is firm.

Beat the butter until smooth. Gradually add the melted chocolate and mix until fully combined. Slowly add the powdered sugar and salt, mixing until smooth. Add 4-5 tablespoons of heavy cream as needed to reach a spreadable consistency.

Place the cheesecake on a serving plate and frost the sides evenly with the chocolate frosting. Press mini chocolate chips and crushed Oreos onto the frosted sides. Using a piping bag, pipe a spiral of frosting on the top center of the cake, working outward toward the edges. Pipe decorative swirls around the outer edge of the top.

Sprinkle remaining mini chocolate chips and Oreo crumbs over the top and garnish with a few whole Oreos.

Refrigerate the finished cake until ready to serve. Because the frosting firms up in the fridge, remove the cake about 45 minutes before serving to make slicing easier. Store the cake covered in the refrigerator and consume within 4-5 days for best taste and texture.