2 tablespoons unsalted butter (1/4 stick), cut into small pieces, plus more for coating the dish
1 pound bittersweet chocolate, finely chopped (about 3 cups)
1/4 cup heavy cream
2 cups Nutella (about 20 ounces)
1/4 teaspoon fine salt
3/4 cup salted, toasted hazelnuts (also known as filberts), skins removed and coarsely chopped
Place a large saucepan filled with about 1 inch of water over medium-low heat until just simmering. Meanwhile, cut an 8-by-16-inch piece of parchment paper. Coat an 8-by-8-inch dish with butter and line it with the piece of parchment so that the 2 long ends hang over the edges. Be sure to fit the parchment as smoothly as possible into the dish, snugly pressing it into the corners; set the dish aside.
Place the measured butter in a large heatproof bowl and set the bowl over (but not touching) the simmering water until the butter has melted. Add the chocolate and cream and stir until the chocolate is smooth and melted, about 4 to 8 minutes.
Remove the chocolate mixture from the heat and stir in the Nutella and salt (the mixture will be very thick). Quickly scrape the mixture into the prepared dish and push it into the corners in a rough, even layer. Sprinkle the nuts evenly over the top. Using your hands, press them into the surface and evenly smooth out the fudge. Refrigerate uncovered until firm, about 1 1/2 to 2 hours.
To remove the fudge, grip the parchment paper hanging over the edges and pull it out of the dish. Transfer to a cutting board and, using a long knife, cut the fudge into 49 (1-1/4-inch) pieces, wiping the knife clean between cuts if necessary. (You may have to let the fudge sit at room temperature until it’s easier to cut.) Let it come completely to room temperature before serving. If desired, place a few pieces of fudge in mini paper baking cups for gifting. Though best eaten right away, the fudge can be stored in an airtight container and refrigerated for up to 3 days.