Line an 8x8- or 9x9-inch square baking pan with parchment paper, leaving overhang on two sides for handles. Lightly grease the parchment and any exposed pan with butter or nonstick spray.

In a heavy 3-quart saucepan, combine the granulated sugar, butter pieces, and evaporated milk. Use a heavy-bottomed pan to help prevent scorching.

Set the pan over medium heat and cook, stirring constantly, until the mixture comes to a full rolling boil with large, steady bubbles across the surface that do not stop when stirred.

Continue to cook at a gentle but steady boil for about 4–5 minutes, stirring constantly, until the mixture reaches 234–240°F (112–116°C) on a candy thermometer (soft-ball stage). If you do not have a thermometer, boil for a full 5 minutes, watching for the mixture to thicken slightly and darken in color.

Remove the saucepan from the heat. Immediately add the chocolate chips, let sit for about 30 seconds to soften, then stir until completely melted and smooth, scraping the sides and corners of the pan.

Stir in the marshmallow creme, vanilla extract, and salt. Continue stirring until the marshmallow is fully incorporated and the mixture looks glossy, thick, and uniform.

Gently fold in the chopped walnuts, if using, until evenly distributed throughout the fudge.

Quickly pour the hot fudge mixture into the prepared pan. Spread it evenly with a spatula and smooth the top as desired. The fudge will begin to thicken as it cools, so work promptly.

Let the fudge cool at room temperature for 30–45 minutes, then transfer to the refrigerator and chill for at least 2 hours, or up to overnight, until firm.

Use the parchment handles to lift the fudge out of the pan onto a cutting board. With a long sharp knife warmed under hot water and dried, cut into 25–36 squares, wiping and re-warming the knife between cuts for the cleanest edges.