Line a 9×13-inch baking pan with parchment paper or heavy-duty aluminum foil. Lightly grease the parchment or foil with butter or nonstick spray to make the fudge easier to remove and slice.

In a large heat-safe bowl, combine the marshmallow creme, semisweet chocolate chips, milk chocolate chips, chopped nuts, and vanilla extract. Set the bowl aside next to the stove so it is ready when the hot sugar mixture is done.

In a large, heavy-bottomed saucepan, combine the granulated sugar, evaporated milk, butter, and salt. Stir over medium heat until the butter melts and the mixture is smooth. Bring to a gentle, steady boil, stirring constantly. Once boiling, continue to cook for 5 full minutes, stirring the entire time to prevent scorching.

As soon as the 5 minutes are up, carefully pour the hot sugar mixture over the bowl with the chocolate chips, marshmallow creme, nuts, and vanilla. Let sit for about 30 seconds, then stir steadily from the center outward until the chocolate and marshmallow are fully melted and the mixture is thick, smooth, and glossy, 2–3 minutes.

Immediately scrape the fudge mixture into the prepared 9×13-inch pan. Use an offset spatula or the back of a spoon to spread it evenly and nudge it into the corners. The top does not need to be perfectly smooth.

Let the pan stand at room temperature for about 30 minutes, then transfer it to the refrigerator. Chill for at least 2 hours, or until the fudge is firm enough to lift from the pan and slice cleanly.

Once firm, use the parchment or foil sling to lift the slab of fudge out of the pan and place it on a cutting board. Slice into 1- to 2-inch squares, wiping the knife with a warm, damp cloth between cuts for neat edges. Serve at cool room temperature.