Line a 9x13-inch baking pan with aluminum foil and butter the foil lightly. Set aside.
In a large heavy saucepan, combine sugar, butter, and evaporated milk. Bring to a full rolling boil over medium heat, stirring constantly.
Continue boiling for exactly 5 minutes, stirring constantly to prevent scorching. Remove from heat.
Quickly stir in chocolate chips until completely melted and smooth.
Add marshmallow creme, vanilla extract, and walnuts (if using). Beat with a wooden spoon until well combined and smooth.
Pour immediately into prepared pan and spread evenly. Let cool completely at room temperature.
Once set, lift fudge out using the foil, cut into 1-inch squares, and store in an airtight container.