Preheat Oven:
Preheat oven to 325°F. Lightly grease a 9-inch springform pan and line it with parchment paper. Set aside.

Make Raspberry Sauce:
In a medium saucepan, combine raspberries, water, sugar, and cornstarch. Cook over medium heat, stirring occasionally, until raspberries break down and sauce thickens, about 20–25 minutes. Set aside to cool slightly.

Prepare Crust:
In a small bowl, mix crushed Oreo cookies with melted butter until fully combined. Press evenly into the bottom of the prepared springform pan. Wrap the pan with two layers of heavy-duty aluminum foil. Set aside.

Melt Chocolate:
In a microwave-safe bowl, combine chocolate chips and ¼ cup of heavy cream. Microwave in 20-second intervals, stirring each time, until melted and smooth. Set aside.

Make Cheesecake Filling:
Using a stand mixer with a paddle attachment, beat the cream cheese until smooth. Add sugar and cocoa powder, mixing until fully incorporated. Add eggs, one at a time, mixing well after each addition. Mix in the remaining ½ cup of heavy cream and vanilla until combined. Gently fold in the melted chocolate until no white streaks remain.

Assemble Cheesecake:
Pour half of the cheesecake filling into the crust. Dollop 4 tablespoons of raspberry sauce over the filling and swirl gently. Add the remaining cheesecake batter and dollop another 4 tablespoons of raspberry sauce on top. Swirl the sauce with a skewer or butter knife, creating a marbled effect.

Bake in Water Bath:
Place the springform pan in a larger pan. Add hot water to the larger pan until it reaches halfway up the springform pan’s sides. Bake for 100–105 minutes, or until the edges are set and the center is slightly jiggly.

Cool and Chill:
Let the cheesecake cool to room temperature on a wire rack (about 1 hour). Refrigerate for at least 4 hours, or overnight, until fully chilled.