Preheat oven to 350°. Spray a 13×9-inch baking pan with cooking spray.
For crust: In a medium bowl, combine flour, sugar, and salt. Using a pastry blender, cut in butter until mixture is crumbly. Press mixture firmly into prepared pan. Bake for 15 to 18 minutes or until lightly browned.
For filling: In a small saucepan, combine butter and chocolate. Cook over low heat, stirring occasionally, until chocolate is melted and mixture is smooth. Let cool for 10 minutes. Spoon mixture into a medium bowl.
Add brown sugar and corn syrup, stirring until combined. Add eggs and vanilla, whisking until combined. Stir in hazelnuts. Spoon mixture over prepared crust. Bake for 25 to 30 minutes or until center is set. Let cool completely before cutting.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/chocolate-hazelnut-bars-recipe/