Line a 9x13-inch or 8x8-inch pan or baking dish (see notes) with parchment paper, leaving an inch or two going over the sides.
Using a heavy medium or large saucepan, melt the butter with the sugar and evaporated milk. Keep the mixture on medium-high heat, until it begins to boil. Keep stirring on medium heat, until it begins to foam, for about 1 to 2 minutes. (If you don’t get the mixture to the right heat, it won’t set. So use the “foam” look as a rule of thumb).
Once the milk and sugar mixture has begun to foam, remove the saucepan from heat. Stir in the chocolate chips and marshmallow creme until they melt. The chocolate will melt pretty quickly, so use a rubber spatula or wooden spoon to combine the melted creme with the chocolate.
Once both have melted into the mixture, stir in ¾ cup of the chopped walnuts, if using. Quickly pour the chocolate mixture into the lined pan or baking dish, and then spread it out evenly. Sprinkle the top of the fudge with the remaining ¼ cup of walnuts, and gently press them into the mixture.
Allow the fudge to cool at room temperature until it sets. Cut into 1x1-inch squares. To prevent your fudge from drying out, keep it stored in an airtight container. You can refrigerate, but it is not necessary.