Preheat the oven to 350°F (175°C). Pulse Oreo cookies in a food processor until fine crumbs. Melt salted butter and mix with cookie crumbs. Press into the bottom of a greased springform pan. Bake for 10 minutes and let cool completely.

Beat cream cheese in a stand mixer until smooth, about 2-3 minutes. Gradually add sugar and cornstarch, blending until fully incorporated. Mix in sour cream and vanilla extract. Add melted dark chocolate and mix well. Finally, add the eggs one at a time, mixing on low speed until just combined.

Pour cheesecake batter into the cooled Oreo crust. Place the springform pan in a larger roasting pan with hot water to prevent cracks. Bake for 1 hour. Turn off the oven and leave cheesecake inside to cool for an additional hour.

Gently release the sides of the springform pan and let cheesecake cool to room temperature on a wire rack. Cover with plastic wrap and refrigerate for at least 8 hours or overnight.

Heat heavy cream in a saucepan until steaming but not boiling. Pour hot cream over chopped dark chocolate in a bowl. Let sit for a few minutes, then stir until smooth. Allow ganache to cool at room temperature.

Pour cooled ganache over the cheesecake top and spread evenly. Optional: whip additional cream for decorative swirls on top and garnish with chocolate shavings. Chill for at least 30 minutes to set before serving.