Line an 8x8-inch (20x20 cm) baking pan with parchment paper or foil, leaving an overhang on two sides to act as handles. Lightly grease the parchment or foil to make it easier to lift out and slice the fudge.

In a medium heavy-bottomed saucepan, combine the semi-sweet chocolate chips, sweetened condensed milk, and unsalted butter. Set over low heat and stir constantly with a rubber spatula or wooden spoon until the chocolate is mostly melted and the mixture is thick and glossy, 4–6 minutes. Keep the heat low to avoid scorching the chocolate.

Remove the saucepan from the heat. Stir in the vanilla extract and fine sea salt until fully combined. Let the mixture sit for 2–3 minutes so it cools slightly; it should still be warm and pourable but not so hot that it instantly melts all of the marshmallows.

Add the 2 cups mini marshmallows and 1 1/2 cups chopped nuts to the warm chocolate mixture. Gently fold until the marshmallows and nuts are evenly coated. A few marshmallows may melt slightly, which is fine.

Scrape the rocky road fudge mixture into the prepared pan and spread it into an even layer, pressing gently into the corners. If using, sprinkle the extra mini marshmallows and/or chopped nuts over the top and press them lightly into the surface so they adhere.

Refrigerate the fudge for at least 2 hours, or until fully set. For the cleanest slices, chill 3–4 hours or overnight.

Use the parchment or foil overhang to lift the slab of fudge out of the pan and onto a cutting board. Warm a large sharp knife under hot water, wipe it dry, and cut the fudge into about 1-inch (2.5 cm) squares, wiping the knife between cuts as needed.

For the best creamy texture, let the fudge sit at room temperature for 10–15 minutes if it has been chilled for several hours or overnight. Serve and enjoy.