1 cup packed hazelnuts, finely chopped to yield 1 cup
Before baking: Center a rack in the oven and preheat the oven to 350 degrees F. Press a sheet of aluminum foil to cover the outside bottom and sides of a 9 by 2-inch square pan.
Lift off the foil, invert the pan, and gently press the foil into the pan to fit the contours.
To make the shortbread: In a food processor, combine the flour, sugar, and salt and process briefly to blend. Scatter the butter pieces over the top and process with 1-second bursts. Add the vanilla and continue with 1-second bursts until the ingredients form small clumps. Stop before the mixture forms a ball. Scatter the clumps over the bottom of the prepared pan, and press with fingertips to line the bottom evenly.
Bake the shortbread until it is pale gold and firm to the touch, 27 to 32 minutes. Let cool in the pan, about 20 minutes.
To make the Heavenly Brownie: Briefly whisk together the flour and salt in a medium bowl to blend; set aside.
In a small, heavy saucepan, melt the butter and chocolate over very low heat, stirring with a silicone spatula until smooth. Remove from the heat and stir the sugar into the chocolate mixture, combining thoroughly. Set aside for 5 to 7 minutes.
In a medium bowl, whisk the eggs until lightly beaten. Add the slightly cooled chocolate mixture to the eggs, stirring to blend. Stir in the vanilla, then the flour mixture. Pour the batter over the shortbread, and spread evenly with a rubber spatula. Sprinkle the nuts evenly over the batter. Gently pat them in place.
Bake the brownie until the top looks slightly cracked about 1 inch from the edge around the pan and feels soft to the touch, 27 – 30 minutes. Don’t overbake; the brownie will firm as it cools. Transfer to a wire rack and let cool completely in the pan, at least 3 hours or up to overnight.
To serve, using the edges of the foil as handles, lift the brownie slab onto a cutting board.
Using a sharp knife, cut the brownies straight down through the shortbread into 1-1/2-inch squares. To store, return the bars to the pan, cover with aluminum foil, sealing the foil around the edges of the pan, and keep at room temperature for up to 3 days.