Prepare the pan (5 minutes).
Line a 9x13-inch baking dish with aluminum foil or wax paper, leaving some overhang for easy removal. Grease lightly with butter.
Mix the base (2 minutes).
In a heavy saucepan, combine sugar, butter, and evaporated milk. Stir over medium heat until sugar dissolves.
Bring to a boil (5 minutes).
Increase heat to medium-high and bring the mixture to a full rolling boil, stirring constantly. Cook for 5 minutes stirring constantly (use a candy thermometer if you like—it should reach 234°F).
Add chocolate (1 minute).
Remove from heat. Add the chocolate chips and stir until completely melted and smooth.
Add marshmallow creme and vanilla (2 minutes).
Stir in marshmallow cream and vanilla until the fudge mixture is silky.
Add nuts (optional, 1 minute).
Stir in chopped walnuts or pecans if using.
Pour and set (2 hours).
Spread evenly into your prepared pan. Let it sit at room temperature until firm, about 2 hours.
Cut and serve.
Lift fudge out of the pan using foil edges. Cut into squares with a sharp knife.