Line a 9-by-13-inch pan with parchment paper or foil and lightly grease it. Measure the chocolate chips, marshmallow creme, vanilla, and optional nuts before cooking so everything can be added promptly.

Place the sugar, butter, and evaporated milk in a heavy saucepan over medium heat. Stir constantly until the butter melts and the mixture reaches a full rolling boil.

Continue boiling for 5 minutes while stirring continuously. If using a candy thermometer, cook until the mixture reaches approximately 234°F (112°C), the soft-ball stage.

Remove the saucepan from the heat immediately. Quickly stir in the chocolate chips, marshmallow creme, vanilla, and optional nuts until the mixture is completely smooth.

Pour the hot fudge into the prepared pan and spread it evenly. Let it cool and set for 2 to 3 hours at room temperature, or refrigerate it for about 1 hour.

Lift the set fudge from the pan using the parchment or foil lining. Cut it into 24 to 36 squares with a sharp knife, wiping the blade between cuts for tidy edges.