• Preheat the oven to 325F (170C) degrees and position the oven rack to about 1/3 from the bottom. 
• Wrap the outside of 9-inch springform pan with aluminum foil so that the bottom and sides are covered. Wrap at least 3 times so that all seems are covered. Lightly grease the pan.

Oreo Cookie Crust

• Crush the Oreo cookies (wafers and filling) in a food processor until fine crumbs. Alternatively, place the cookies in a freezer bag and crush with a rolling pin.
• Mix the cookie crumbs with the melted butter.
• Press the mixture into the bottom and about 1/2 to 2/3 of the way up the sides of the springform pan.
• Bake in the oven for 10 minutes. Remove from the oven (keep the oven turned on).

Oreo Cheesecake Filling

• In a very large bowl, beat the cream cheese, granulated sugar and cornstarch until creamy and smooth. Turn off the mixer and scrape down the sides and bottom of the bowl to ensure there are no lumps.
• Beat in the sour cream and vanilla extract.
• With the mixer on low speed mix in the eggs one at a time until just combined. 
• Fold in the chopped Oreo cookies using a rubber spatula or wooden spoon. I usually fold in about 2/3 of the chopped Oreos.

Baking & Cooling

• Pour the batter into the prepared pan. If you reserved some Oreo pieces, add them to the top of the cheesecake and press them down slightly so they are covered in cheesecake batter.
• Place the cheesecake in the middle of a large roasting pan.
• Pour about 1/2 - 1 inch (1-2 cm) of boiling water into the roasting pan.
• Place the roasting pan with the cheesecake inside in the oven and bake for 50-60 minutes, or until the top looks just set except for in the very middle and if you give the pan a gently nudge there's a slight wobble in the middle of the cheesecake. 
• Cool at room temperature until the water in the water bath is lukewarm. Then remove the cheesecake from the roasting pan and continue cooling until the sides of the springform pan are room temperature. 
• Cover and refrigerate for at least 6 hours or overnight. 

Topping

• Chop the chocolate into very small pieces and place in a heat-proof bowl.
• Heat 1/2 cup cream until almost boiling (either in the microwave and in a saucepan over low heat).
• Pour the hot cream over the chopped chocolate and let sit for 2 minutes. Then whisk until smooth. If not all the chocolate is quite melted yet, return to the microwave for 30 seconds on medium power (not high power) and whisk until smooth.
• Remove the cheesecake from the fridge. Gently trace around the outside of the cheesecake with a thin knife, then unclamp the outer ring of the springform pan. If there's condensation on top of the cheesecake, lightly blot with a paper towel or clean tea towel.
• Pour the chocolate over the cheesecake, allowing it to drip over the edges.
• Optionally, decorate the cheesecake with additional whipped cream.