Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides to act as handles. Lightly mist the parchment with cooking spray if sticking is a problem in your kitchen.
In a medium heat-safe bowl, combine the semi-sweet chocolate chips, Nutella, sweetened condensed milk, and butter pieces.
2 cups Nutella, 2 cups semi-sweet chocolate chips, 14 oz sweetened condensed milk, 4 tablespoons unsalted butter
Microwave the mixture on medium power for 30 seconds, then stir well. Continue heating in 20–30 second bursts, stirring after each, until the chocolate chips are mostly melted and the mixture is thick, smooth, and glossy. Do not let it boil; reduce power or time if it starts to bubble.
Alternatively, place the chocolate chips, Nutella, sweetened condensed milk, and butter in a medium saucepan over low heat. Stir constantly with a silicone spatula until completely smooth and melted. Keep the heat low to maintain a silky texture.
2 cups Nutella, 2 cups semi-sweet chocolate chips, 14 oz sweetened condensed milk, 4 tablespoons unsalted butter
Remove the melted mixture from the heat and immediately stir in the vanilla extract and fine sea salt until fully incorporated. The mixture will be very thick, like warm Nutella frosting.
1 teaspoon pure vanilla extract, 1/4 teaspoon fine sea salt
Quickly scrape the fudge mixture into the prepared pan. Use an offset spatula or the back of a spoon to spread it into an even layer, pressing it gently into the corners. Work fairly quickly, as it will begin to firm up as it cools.
While the fudge is still warm and soft on top, sprinkle on any desired toppings such as chopped toasted nuts, flaky sea salt, mini chocolate chips, or crushed cookies. Press them very lightly into the surface so they adhere without sinking.
1/4 cup chopped toasted hazelnuts or mixed nuts, 1 to 2 tablespoons flaky sea salt, 1/4 cup mini chocolate chips or white chocolate chips, 1/4 cup crushed wafer cookies or Biscoff cookies
Cover the pan loosely with foil or plastic wrap and refrigerate for at least 2–3 hours, or until firm enough to cut. For the cleanest slices, chill overnight. For a faster set, place in the freezer for 45–60 minutes.
Use the parchment overhang to lift the fudge out of the pan onto a cutting board. Run a sharp knife under hot water, wipe it dry, and cut the fudge into approximately 1-inch squares, reheating and wiping the knife between cuts for neat edges.
Arrange the fudge squares on a platter for serving or layer them in an airtight container with parchment between layers. Serve slightly chilled or at cool room temperature.