Preheat the oven to 350 F. Place the hazelnuts in a single layer on a lined baking sheet. Roast until they're golden, about 7-10 minutes. (Be careful not to over-roast or they will be bitter). If they have skins on them, place them in a clean dish towel and rub them against each other to remove the skins.
Place nuts in a food process with the sugar and blend until the mixture comes together in a mass (resembling very thick peanut butter), about 5 minutes. Add the melted cocoa butter or butter and blend until combined, scraping down the sides as needed. Add the melted chocolate and pulse until the mixture is combined and smooth, scraping down the sides as needed. Scrape the mixture into a bowl, then cover and refrigerate until it is firm. At this point you can shape it into a log, wrap and store it in the fridge or just leave it in the bowl and scoop out as needed. Made with cocoa butter it will keep for several weeks in the fridge. Made with butter it will keep for a couple of weeks.Let the nougat come to room temp before using so it will be softer and malleable. Makes about 19 ounces (540 grams) of nuss-nougat rohmasse.