Line an 8-inch square pan with parchment or foil, leaving overhang. Lightly butter the lining.
Combine sugar, butter, and evaporated milk in a heavy saucepan. Set over medium heat and stir until melted.
Bring to a steady boil, stirring often. Cook 5 minutes, or until 234–238°F on a candy thermometer.
Remove from heat. Add chocolate chips and stir until completely smooth and glossy.
Fold in marshmallow creme and vanilla until evenly combined. Mix in salt if using.
Stir in nuts, if desired, then scrape the fudge into the prepared pan and smooth the top.
Let cool at room temperature until firm, 2–3 hours. Lift out and cut into small squares.