Place the finely chopped dark chocolate and softened butter into a heat-proof mixing bowl.
Pour the heavy cream into a small saucepan and bring to a gentle simmer over medium heat. Do not let it reach a full, rolling boil, as this can scald the cream.
Once small bubbles form around the edges of the cream, remove the pan from the heat immediately.
Pour the hot cream over the chopped chocolate and butter. Let the mixture sit undisturbed for 3 minutes to allow the heat to penetrate the chocolate evenly.
Using a silicone spatula, stir gently in small circles starting from the center of the bowl until the mixture is glossy, smooth, and emulsified.
Fold in half of the chopped toasted hazelnuts into the ganache mixture.
Cover the bowl with plastic wrap and refrigerate for 2 to 3 hours, or until the mixture is firm enough to scoop.
Line a baking sheet with parchment paper. Using a melon baller or two teaspoons, scoop out small portions of the ganache and roll them quickly between your palms to form spheres.
Roll each sphere in the remaining chopped hazelnuts or in the unsweetened cocoa powder until evenly coated.
Place the finished truffles on the prepared baking sheet and refrigerate for another 30 minutes to set before serving.