Line an 8- or 9-inch square baking pan with parchment paper or heavy-duty aluminum foil, leaving an overhang on two sides to create a sling. Lightly grease the parchment or foil with butter or a quick spray of cooking oil. Measure out the mini marshmallows and chopped nuts into separate bowls and set aside so they are ready to add quickly later.

In a medium heavy-bottomed saucepan over low heat, combine the semi-sweet chocolate chips, sweetened condensed milk, and butter. Stir constantly with a silicone spatula or wooden spoon as the chocolate begins to melt, keeping the heat low to prevent scorching or seizing. Continue stirring until the mixture is completely smooth, thick, glossy, and lump-free. Remove the pan from the heat and stir in the vanilla extract and fine sea salt until fully incorporated.

As an alternative to the stovetop, place the semi-sweet chocolate chips, sweetened condensed milk, and butter in a large microwave-safe bowl. Microwave on 50–70% power in 20–30 second bursts, stirring well after each interval, until the mixture is fully melted and smooth. Do not rush this—short bursts at reduced power help prevent the chocolate from overheating and turning grainy. Once smooth, stir in the vanilla extract and fine sea salt.

Let the hot chocolate mixture sit off the heat for about 2 minutes; it should still be thick and pourable but slightly cooler. Add the mini marshmallows and chopped nuts to the pan. Using a spatula, gently fold them into the chocolate mixture until everything is well coated, taking care not to overmix so that some marshmallow pieces remain visible for a classic rocky road look.

Scrape the rocky road fudge mixture into the prepared pan, spreading it evenly and pressing it gently into the corners. Use the back of a spoon or an offset spatula to smooth the top. If using, sprinkle the additional chocolate chips or chunks and a few extra nuts over the surface and press them lightly so they adhere.

Place the pan in the refrigerator and chill for at least 2 hours, or until the fudge is firm to the touch in the center. For very clean slices, you can chill it closer to 3 hours or until fully set.

Use the parchment or foil overhang to lift the firm fudge out of the pan and onto a cutting board. With a sharp knife, cut into about 36 small squares (or 25 larger pieces if preferred). For extra-neat edges, warm the knife under hot water, wipe it dry, slice a row, then wipe and repeat. Serve slightly chilled or at cool room temperature.

Store fudge squares in an airtight container. Keep at cool room temperature for up to 3 days, in the refrigerator for up to 2 weeks, or freeze for up to 3 months. If layering pieces, separate layers with parchment or wax paper. Thaw frozen fudge briefly at room temperature until slightly softened before serving.