1Position a rack in the center of the oven and preheat the oven to 350ºF. Lightly butter an 8-inch square baking pan. Line the pan on the bottom and up two sides with a strip of parchment paper or nonstick aluminum foil. Dust the inside of the pan with flour and tap out the excess flour.
2Whisk the flour, cocoa, and salt together in a small bowl. Melt the butter in a medium saucepan over medium-low heat. Remove from the heat and add the chocolate. Let stand for 3 minutes. Whisk until the chocolate melts. Add the chocolate-hazelnut spread and whisk until smooth, followed by the sugar and vanilla. One at a time, whisk in the eggs, waiting until each egg is absorbed before adding the next. Stir (do not whisk) in the flour mixture, followed by the hazelnuts. The batter will be thick. Spread evenly in the prepared pan.
3Bake until a wooden toothpick inserted in the center of the pastry comes out with a moist crumb, 25 to 30 minutes. Let cool completely in the pan on a wire rack. Run a knife around the pan to loosen the pastry. Use the paper “handles” to lift the pastry out of the pan in one piece. Cut into 9 brownies. (The brownies can be wrapped in plastic wrap and stored at room temperature for up to 2 days.)
Note: To toast and skin hazelnuts, spread the hazelnuts in a rimmed baking sheet. Bake in a preheated 350ºF oven, stirring occasionally, until the skins are cracked and the nuts look light brown underneath the skins, 10 to 15 minutes. Transfer the hazelnuts to a kitchen towel and let stand until cooled, about 20 minutes. Rub the nuts against each other in the towel to remove the skins. Don’t worry if some of the skins are stubborn and don’t come off. To “chop” the nuts, it is easiest to actually crush them on a carving board with a flat meat tenderizer or a moka (Italian coffee) pot.