Preheat the oven to 325°F.

Butter an 8-inch square pan and dust with flour, tapping out any excess. Set pan aside.

Combine the flour and salt in a small bowl. Coarsely chop the hazelnuts.

Lightly beat the eggs with a fork in a large bowl until yolks and whites are combined.

Add the Nutella, brown sugar, and vanilla and stir vigorously with a rubber spatula until well combined.

Add the melted butter and mix until well combined. Add the flour mixture and stir until blended. Fold in the hazelnuts.

Pour the batter into the prepared pan and smooth into an even layer.

Bake the brownies on middle rack of oven until a toothpick inserted in the center comes out with a few moist crumbs attached, about 45 minutes.

Check to see that brownies are done. Remove from oven or add time as needed.

Set brownies on a cooling rack and cool completely in the pan, about 2 hours.

Cut the brownies into 16 squares. Serve immediately.