Begin by preheating your oven to 350°F (175°C). In a mixing bowl, combine the finely crushed Oreos and melted butter. Stir until the crumbs are evenly coated with butter. Press this mixture firmly into the bottom of a 9-inch pie dish or your desired serving cups. Bake for 10 minutes, then set aside to cool.
In a large mixing bowl, combine the softened cream cheese and 2 tablespoons of granulated sugar. Use an electric mixer to beat them together until smooth and creamy. Add in the Nutella, cocoa powder, and vanilla extract, mixing until well combined.
In another bowl, whip 1 1/3 cups of heavy cream with 2 tablespoons of granulated sugar until soft peaks form. Be careful not to overwhip! Gently fold the whipped cream into the Nutella cream cheese mixture, ensuring you maintain the light and airy texture.
Once your Oreo crust has cooled, spoon the Nutella cheesecake filling over the crust, smoothing it out with a spatula.
Cover the dish with plastic wrap and refrigerate for at least 2 hours, or until the mousse is set.
Before serving, whip the remaining 2/3 cup of heavy cream with 1 1/2 tablespoons of granulated sugar until soft peaks form. Spread or pipe this whipped cream over the set mousse for a delightful finishing touch.
Top your mousse with optional toasted, chopped hazelnuts and chocolate shavings or sprinkles for an extra layer of flavor and decoration. Serve chilled and enjoy each creamy, chocolatey bite!