Lightly grease a 9x13-inch baking pan and line it with parchment paper, leaving some overhang on the sides. Lightly spray the paper with cooking spray and set aside.

In a heavy-bottomed 3-quart saucepan, combine the sugar, butter, and evaporated milk. Bring the mixture to a full rolling boil over medium heat, stirring gently and constantly to prevent scorching. Boil for 5 minutes or until a candy thermometer reaches 236-238°F.

Remove the pot from the heat and immediately add the chocolate chips. Stir until smooth, then add the marshmallow creme and mix vigorously until well blended. Stir in the vanilla extract and walnuts, if using.

Quickly pour the hot fudge into the prepared pan and spread it into an even layer with a spatula. Let it set at room temperature for at least 4 hours, then place it in the fridge for 1-2 hours.

Use the parchment paper to lift the fudge out of the pan and transfer it to a cutting board. Cut into small squares, serve, and enjoy!