Preheat your oven to 350°F (175°C). In a bowl, crush Oreo cookies until finely ground, then mix with melted butter and granulated sugar until combined. Press this mixture firmly into the bottom of an 8-inch springform pan. Bake for 12-14 minutes until set, then cool completely.

In a microwave-safe bowl, melt the chocolate chips until smooth, then set aside to cool slightly. In a mixer, whisk together sugar and cornstarch until well blended. Add the cream cheese and beat on medium speed until smooth and creamy. Gradually incorporate the eggs, sour cream, and vanilla extract.

Separate 2 cups of this batter and mix in the melted chocolate until thoroughly combined.

Spoon alternating dollops of the vanilla and chocolate cheesecake batter into the cooled crust, creating a marbled appearance. Swirl gently with a knife.

Lower the oven temperature to 300°F (150°C). Place the cheesecake pan on a water-filled baking sheet and bake for 15 minutes, then reduce temperature to 225°F (110°C) and continue baking for about 2 hours. Do not open the oven door.

Turn off the oven, cool the cheesecake inside for 1 hour with the door slightly ajar. Cool for another hour, then allow to cool at room temperature for 2 hours before refrigerating for at least 3-4 hours, preferably overnight.