Grease the bottom of an 8-inch square or comparable sized pan with butter. Line the bottom and sides of the pan with aluminum foil, allowing 2 to 3 inches of foil to extend over sides. Butter the foil and set aside.
Combine sweetened condensed milk, chocolate morsels, and hazelnut spread in a large saucepan. Cook over medium heat and stir until melted and smooth, about 8 minutes. Remove from heat and stir in 3/4 cup of the hazelnuts, the vanilla, and salt. Spoon fudge into prepared pan and quickly spread out evenly with a spatula. Sprinkle the top of the fudge with the remaining hazelnuts while it is still soft.
Cover and chill one hour until firm. Using foil sides as handles, lift from the pan. Peel away the foil and cut fudge into small squares. Store the candy covered in the refrigerator and it will keep for up to one week.