1. Preheat the oven to 350ºF. Grease a springform pan and wrap the outside of the pan in foil. Set aside. 

Chocolate Cookie Crust

1. In a bowl, mix crushed cookies, butter and sugar. Transfer to the pan and press evenly into the bottom using the bottom of a glass. Place the crust in the oven and bake for 10 minutes until the crust is slightly brown around the edges. Remove from oven and let it cool. 

Cheesecake Filling

1. In a bowl, mix the cream cheese, sugar, cornstarch and salt until combined and creamy. Stir in sour cream, melted chocolate and vanilla then mix until combined. Add one egg at a time and stir to combine. Make sure to scrape the sides of the bowl and mix until smooth and creamy. Pour the cheesecake batter over the cooled crust.  
2. Transfer the springform pan to the middle of a large roasting pan. Carefully fill the roasting pan with about an inch of hot water. This will help the cheesecake remain creamy without cracks. Bake the cheesecake for 55-60 minutes.  
3. Remove cheesecake from oven. The cheesecake is done when the outer rim looks set but the middle jiggles when you gently shake the pan. Let the cheesecake cool completely on a cooling rack. This step is crucial for achieving the perfect cheesecake. Place in the refrigerator for at least 4 hours or up to 72 hours.