Add the butter, sugar, marshmallows, and evaporated milk to a heavy bottom pot. If you don't have one, you can use a regular pot. Set the heat on the burner to medium. Stir constantly and heat the mixture to 234F/112 C.
Once the sugar mixture reaches 234F/112 C. Remove it from the heat and add in the chocolate chips. Stir, stir, stir. Once melted, add in the vanilla extract. Keep stirring for a few minutes until it loses some of its shine and is duller looking. Then add in the optional chopped walnuts.
Scoop the hot fudge out of the pot and into a baking pan (11x7 or 9x9 inch pan) lined with parchment paper, or spray it with cooking spray.
Spread the fudge out with a spatula until it is level. Then let it sit at room temperature for an hour or two to set. You can also leave it overnight. If you do, after an hour or two, I recommend, covering it with plastic wrap, which helps it to no dry out. Not necessary, but something, I like to do.
And when the fudge becomes firm, it is ready to be cut. It is common to just cut it up into little cubes. Enjoy!