Lightly butter a deep 9 inch square baking dish and line with parchment paper, ensuring that it comes up and over the sides of the dish - this will make it easier to remove from the baking dish once it’s ready to serve. Place the baking dish in the fridge.
Add all but 2 of the dark chocolate digestive cookies and the original digestive cookies into a food processor and pulse until fine. Add the rolled oats, melted butter, and 1 tablespoon of Nutella and pulse again until well combined and the crust starts coming together.
Pours the cookie crumbs mixture into the chilled baking dish, spreading and patting it evenly and into all the corners. Chill in the refrigerator until the filling is ready.
Add the dark chocolate to a small microwave-safe bowl and melt in the microwave in 30 second intervals until melted or use a double boiler. Set aside to cool for a few minutes while continuing.
In a large bowl add the cream cheese and confectioner's sugar. Use an electric mixer on low speed and combine until all confectioner's sugar is incorporated. Once it is all mixed together add in the melted chocolate, Nutella, and the milk. Using an electric mix the cheesecake batter until well mixed. You can also use a stand mixer and the bowl of your stand mixer with the paddle attachment when making the creamy cheesecake filling.
Remove the chilled cookie crumb crust from the fridge and pour the chocolate cheesecake filling into the crust. Use a spatula to spread the cheesecake filling. Crush the reserved cookiiese and mix with the hazelnuts if using them and sprinkle on top of the cheesecake filling.
Cover with plastic wrap and chill for at least 4 hours or preferably overnight.
When completely chilled, remove from the baking dish using the parchment paper. Place the no-bake chocolate cheesecake flat onto a chopping board. Use a sharp knife to cut into slices or squares.