Preheat oven to 325 degrees F.
Line a baking sheet with parchment paper, and spread hazelnuts in a single layer on the sheet. Place in the oven, and toast until nuts smell sweet, about 10 to 12 minutes. Wrap the warm nuts in a coarse-textured kitchen towel. Rub to remove skins from nuts, and discard skins. Coarsely chop hazelnuts, and set aside.
In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites on low speed until foamy. Add sugar gradually, beating on medium-high, until stiff peaks form and the meringue is stiff and shiny, 3 to 5 minutes. (You can also beat egg whites by hand using a balloon whisk; the beating times will be approximately twice as long).
Fold in the chopped hazelnuts, vanilla extract, and salt using a large rubber spatula.
Transfer mixture to a shallow, heavy-bottomed saucepan. Set over medium-low heat, and cook, stirring constantly, until the mixture is light brown and pulls away from the sides of the pan, about 15 to 20 minutes. Remove from heat. Transfer to a medium bowl to cool slightly, about 5 minutes. Fold in chopped chocolate.
Using 2 spoons, drop a heaping tablespoon of batter onto a baking sheet lined with a nonstick baking mat or parchment paper. Repeat until no dough remains, spacing them 1 1/2 inches apart. Bake until cookies are firm to the touch but still soft and chewy inside, about 25 to 30 minutes. Remove to a wire rack, and cool.
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