While the cookie crust bakes and cools, make the cheesecake filling.
Add 1 1/4 cups of dark chocolate chips and 1/4 cup of Nutella into a medium-sized bowl. Pour 1/3 cup of hot coffee or hot water into the bowl, making sure all the chocolate chips are submerged. Let the mixture sit for 1 minute, then stir until the mixture is smooth. If there are any bits of unmelted chocolate, heat the bowl in the microwave for 15 seconds, then let it sit for a minute and stir until smooth. Set aside.
In a small bowl, whisk together 1 cup of sugar and 3 Tbsp of cornstarch. This helps prevent the cornstarch from clumping.
In a large mixing bowl or the bowl of a stand mixer fit with a paddle attachment, beat 3, 8 oz. packages of room temperature cream cheese and the sugar / cornstarch mixture on a low speed until the mixture is completely smooth. Scrape the sides and bottom of the bowl with a rubber spatula to make sure everything is properly combined.
Mix in 4 eggs on a low speed, one at a time. Scrape the sides and bottom of the bowl with a rubber spatula as needed. Then add 1/2 cup sour cream and 1 tsp vanilla extract. Mix until just combined.
Next, mix the chocolate / coffee mixture into the cream cheese mixture on a low speed until combined.
The chopped Oreos can either be folded directly in the batter or added as a layer in the middle of the cheesecake. If you want to add them as a layer in the middle, pour half the batter into the Oreo crust, add a layer of chopped Oreos, then cover them with the rest of the cheesecake batter. NOTE: If you use an 8-inch pan, it will be filled up to the very top of the pan! But don't worry, cheesecakes don't really rise as they bake, so it shouldn't overflow in the oven.
Double check that the oven temperature has been lowered to 300 F / 150 C, then place the cheesecake pan on a large cookie sheet and bake the cheesecake for 12 minutes.
Without opening the oven door, reduce the oven temperature to 225 F / 110 C and bake for an additional 115 minutes. It sounds crazy, but low and slow is the best way to bake up a perfect, crack-free cheesecake that doesn't sink.
Once the cheesecake is done baking, turn the oven off and let the cheesecake sit in the oven for 1 hour without opening the door.
After 1 hour of cooling in the oven, crack the oven door open with a wooden spoon and let the cheesecake cool in the oven for another hour.
Next, remove the cheesecake from the oven and let it continue to cool at room temperature for 2 hours. Once it reaches room temperature, cover the top of the cheesecake with plastic wrap and refrigerate the cheesecake for an additional 3-4 hours or overnight (preferred). Store the cheesecake in the fridge until you're ready to serve it.
When you're ready to enjoy this Nutella Oreo cheesecake, remove it from the fridge and carefully release it from the pan.
Drizzle warm Nutella over the top of the cheesecake and top with whipped cream swirls and chopped Oreos. Feel free to get creative with how you decorate it! You can also cover the entire top of the cheesecake with warm Nutella.
Use a sharp, warm knife to cut into the chilled cheesecake. Clean the knife after each cut to get perfect slices. Leftover cheesecake can be kept in the fridge for up to 5 days in an airtight container.