Line an 8-inch or 9-inch square baking pan with foil or parchment, leaving enough overhang on two sides to lift out the set fudge. Lightly butter the lining and set the prepared pan aside.

Place the granulated sugar, unsalted butter, and evaporated milk in a heavy-bottomed saucepan. Warm over medium heat, stirring frequently, until the butter melts and the mixture reaches a full rolling boil.

Boil the mixture for about 5 minutes, stirring constantly so the syrup does not scorch on the bottom. If using a candy thermometer, cook until it reaches approximately 234 to 238°F, the soft-ball stage.

Remove the saucepan from the heat immediately after the boiling stage. Quickly add the semisweet chocolate chips, marshmallow creme, and vanilla extract, stirring until the mixture is completely smooth and blended.

Fold in the chopped pecans or walnuts if using. Stir only until the nuts are distributed evenly because the fudge begins to thicken quickly.

Pour the hot fudge into the prepared pan and smooth the top with a spatula. Cool at room temperature, then refrigerate until firm, allowing about 2 to 3 hours for the fudge to cool and set completely.

Lift the fudge from the pan using the foil or parchment overhang. Let it stand briefly to soften slightly, then cut it into small squares with a sharp knife.