Preheat the oven to 325 degrees Fahrenheit.
Add the Oreos and hazelnuts to the bowl of a food processor. Pulse until finely ground. Add the butter and salt, and pulse again just until combined and moist.
Press the mixture into the bottom and along the sides of a 9-inch tart pan.
Bake for 6-8 minutes, and set aside to cool.
In a medium heatproof bowl, add the chocolate. Set aside.
In a small saucepan over medium heat, bring the heavy cream, sugar, and butter to a low boil. Remove the pan from the heat, and pour the mixture over the chocolate. Let stand for 1 minute.
Use a rubber spatula to stir until everything is melted and smooth. Add the vanilla and hazelnut butter, and stir again until smooth. Note: If the mixture is not completely melted, place it in the microwave for 30 seconds, and stir again.
Pour the filling into the prepared crust. Refrigerate for at least 4 hours or up to overnight.
In a small saucepan over medium heat, melt the butter. Add the brown sugar and salt, and stir to combine.
Add the hazelnuts, and stir for 2-3 minutes or until lightly toasted and fragrant.
Place the nuts on a silicone mat or parchment paper to cool completely.
Sprinkle the nuts and salt on top of the chocolate tart, slice, and serve chilled.
Store in the fridge for up to 1 week.
