Preheat the oven to 350 degrees F and grease a 9 x 9 baking pan.
In a medium bowl, whisk the flour, cocoa powder, and salt.
In a larger, microwave-safe bowl, combine the chocolate, Nutella and butter. Heat in the microwave for 30 second intervals, stirring after each interval, until the chocolate, Nutella, and butter are completely melted and smooth. Add the sugars, whisking until fully incorporated, then add the eggs and vanilla.
Fold in the flour mixture until just combined, followed by the hazelnuts.
Spread the batter evenly into the prepared pan and bake until cooked through, about 30 minutes. Let the brownies cool, then cut them into squares and serve.