Adjust an oven rack to the center position, then preheat the oven to 325 degrees F.
Line the bottom of a 9-inch springform pan with parchment paper.
Add the graham crackers to a food processor and pulse until crumbs form.
In a bowl, add the graham cracker crumbs, then stir in 1 stick of the melted butter and 3 tablespoons of the granulated sugar.
Firmly press the crust mixture into the bottom of the prepared pan, bringing it up about 1 inch along the sides.
Bake the crust for 10 minutes, then transfer the pan to a wire rack and let it cool completely.
Increase the oven temperature to 350 degrees F.
Pour the Nutella into a small, microwave-safe bowl and microwave in 30-second intervals until the consistency is sauce-like.
In a large bowl, add the Nutella, then whisk in the large eggs, 1 at a time.
Add the flour to the Nutella mixture and stir to combine.
Let the Nutella mixture cool slightly.
Fold the chopped chocolate-hazelnut bar into the Nutella mixture.
Pour the brownie batter into the prepared pie crust and bake, about 20-25 minutes.
While the brownies are baking, in a medium saucepan, add the remaining granulated sugar, the cornstarch, 1/4 cup of the cocoa powder, and the salt and whisk to combine.
Add the half-and-half and the egg yolks to the pudding mixture and whisk to combine. Cook over medium heat, whisking constantly, until it has thickened and is pudding-like, about 10-12 minutes.
Transfer the saucepan from the heat and stir in the remaining butter, the chopped bittersweet chocolate, and 1 teaspoon of the vanilla extract.
Let the pudding cool.
Spread the cooled pudding into an even layer over the brownie.
Place plastic wrap directly over the surface of the pudding layer and refrigerate until it is chilled all the way through, for at least 4 hours and up to overnight.
In a large bowl, add the heavy whipping cream, the confectioners' sugar, and the remaining vanilla extract and beat with an electric hand mixer until soft peaks form.
Discard the plastic wrap and spread the whipped cream in an even layer over the pudding layer.
Sprinkle the bars dusted with the remaining cocoa powder and sprinkled with the chocolate chip cookie crumbs, then transfer it from the springform pan.
Cut the bars into squares and serve.