Lightly grease an 8x8-inch (20x20 cm) square pan with a thin layer of butter or nonstick spray. Line it with parchment paper, leaving some overhang on two opposite sides to create handles. Smooth the parchment into the corners for sharp, neat edges.

In a medium, heavy-bottomed saucepan, add the semi-sweet chocolate chips, Nutella, butter pieces, and the entire can of sweetened condensed milk.

Set the saucepan over low to medium-low heat. Stir continuously with a silicone spatula or wooden spoon until the mixture melts and becomes thick, glossy, and completely smooth. Keep the heat gentle to avoid scorching. (Alternatively, melt in a large microwave-safe bowl in 20–30 second bursts, stirring after each, until smooth.)

Remove the pan from the heat. Stir in the vanilla extract and fine sea salt until fully combined. Taste a tiny bit carefully (it will be warm) and adjust the salt if you want a slightly less sweet flavor.

If using chopped toasted hazelnuts, pecans, or mini chocolate chips inside the fudge, gently fold them into the warm mixture now. Work fairly quickly, as the fudge will thicken as it cools.

Pour the warm Nutella fudge mixture into the prepared pan. Use an offset spatula or the back of a spoon to spread it into an even layer, pushing it into the corners.

While the surface is still soft, sprinkle with flaky sea salt, extra chopped nuts, and/or mini chocolate or white chocolate chips if desired. Press toppings very gently so they adhere without sinking too deeply.

Let the pan cool at room temperature for 10–15 minutes. Transfer to the refrigerator and chill for at least 2 hours, or until the fudge is firm enough to cut cleanly. For very sharp edges, chill 3–4 hours or overnight.

Use the parchment handles to lift the slab of fudge out of the pan onto a cutting board. Warm a sharp knife under hot water, wipe it dry, and cut the fudge into 1–1.5 inch squares, wiping and rewarming the knife between cuts for the neatest edges. Serve chilled or at cool room temperature.

Store fudge squares in an airtight container in the refrigerator for up to 2 weeks, with parchment or wax paper between layers. For longer storage, freeze in a freezer-safe container (with parchment between layers) for up to 3 months. Thaw in the fridge for a few hours or at room temperature for 20–30 minutes before serving.