Preheat the oven to 350°F.
For the Salted Butter Shortbread: Combine the flour, brown sugar, and salt in bowl of a food processor and pulse a few times to combine. Add the butter and pulse until the mixture is full of pea-size lumps. (If you don’t have a food processor, just combine the mixture in a bowl. Using a fork and knife, cut the butter into the flour to create pea-size lumps. Or just use your fingers!)
Sprinkle the mixture into an ungreased 13″x9″x2″ baking pan; press the mixture evenly into the pan with your hands (or a metal spatula). Place in the oven and bake on the middle rack for 20 minutes, or until golden. Remove from the oven and let cool slightly while making the brownie layer. Keep the oven set to 350°F.
For the Hazelnut Nutella Brownies: Melt the butter and the chocolate in a medium saucepan over low heat, stirring constantly. Remove from the heat and let cool slightly.
Pour the mixture into a large mixing bowl. Add the sugar and beat well with electric beaters (or a whisk). Add the eggs, one at a time, beating until thoroughly combined. Add the vanilla and the Nutella and continue beating.
Stir in the flour and the salt, until no streaks remain. Stir in the hazelnuts. Pour the mixture over the shortbread base and smooth the surface with a spatula.
Bake on the middle rack for 35 to 40 minutes or until a toothpick comes out with moist crumbs attached. (The middle of the brownies may crack a bit–this is normal.) Cool completely on a cooling rack before slicing.
TO CUT: Using a sharp knife, cut around the edges of the brownies to loosen them from the pan. Using one hand on the bottom of the pan and one hand on the brownies, carefully flip the pan upside down onto a cutting board. Tap the pan a few times, then remove it–the brownies should be shortbread-side up on the cutting board.
Flip them over so the shortbread side is on the cutting board and the brownies are on top. Dip a sharp knife into hot water and carefully cut the brownies into 24 bars (cleaning the knife and re-dipping it into the water between cuts). It’s important to have a sharp knife for this process to cut through the hazelnuts!
Store the brownies in an airtight container at room temperature for up to 5 days (or freeze).
*To roast the hazelnuts, place them on a baking sheet and roast them in a 350°F oven for 5 to 7 minutes or until lightly browned and the skins are beginning to split. Dump the hazelnuts into a bowl and using a kitchen towel (so you don’t burn your fingers!), rub the skins off and discard them.