In a medium saucepan over medium heat, combine the frozen raspberries, water, sugar, and cornstarch. Stir to combine. Cook, stirring occasionally, until the raspberries break down and the sauce thickens, about 20-25 minutes. Set aside to cool slightly.

In a small bowl, combine the crushed Oreo cookies and melted butter, stirring until the crumbs are fully coated. Press the mixture evenly into the bottom of the prepared springform pan. Wrap the bottom and sides of the pan with two layers of heavy-duty aluminum foil. Set aside.

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

To a microwave-safe bowl, add the chocolate chips and ¼ cup of the heavy cream. Microwave in 20-second intervals, stirring between each, until the chocolate is melted and the mixture is smooth. Set aside.

In the bowl of a stand mixer with a paddle attachment, add the cream cheese. Mix until smooth. Add the sugar and cocoa powder, beating until fully incorporated.

Add the eggs, one at a time, mixing well between each addition. Pour in the remaining ½ cup of heavy cream and vanilla. Mix until combined.

Gently fold in the melted chocolate mixture. Stir until no white streaks remain.

Pour half of the cheesecake filling into the crust. Dollop 4 tablespoons of raspberry sauce over the filling. Carefully add the remaining cheesecake batter, then dollop 4 more tablespoons of the raspberry sauce on top. Use a butter knife or skewer to gently swirl the sauce into the filling, creating a marbled effect without disturbing the crust. Do not fully mix the sauce into the filling.

Place the springform pan into a larger pan, such as a roasting pan. Pour hot water into the larger pan until it reaches about halfway up the side of the springform pan.

Bake for 100-105 minutes (1 hour 40 to 45 minutes), or until the sides are set and the center is slightly jiggly.

Remove the cheesecake from the oven and allow it to cool on a wire rack until it reaches room temperature (about 1 hour). Then, transfer it to the refrigerator to chill for at least 4 hours, or overnight.

Garnish with mint leaves. Serve with the remaining raspberry sauce.