In a heatproof bowl, combine chocolate chips, butter, and marshmallow cream. Set aside.
In a large (see note) saucepan, combine evaporated milk and sugar. Stir until well mixed.
Heat to a rolling boil (see note) and continue to boil for 6 minutes, stirring continuously.
Pour over chocolate chips, butter, and marshmallow cream. Stir until smooth and no more lumps remain.
This should be done as quickly as possible to avoid the fudge starting to set.
Add vanilla and nuts (if desired) and stir until combined.
Pour into a greased or lined 9x13 pan.
Refrigerate overnight, or at least until set (about 4 hours).
A standard can of marshmallow cream is usually 7 ounces, I don't worry about measuring it. I get the majority out of the can and leave whatever is left, which is close to 2 ounces. Nuts are obviously optional and can be replaced with whatever mix-ins you like (chopped peppermint, cherries, etc.) Use at least a 3-quart saucepan for the evaporated milk and sugar mixture to avoid boiling over. A rolling boil is when the mixture is boiling and doesn't stop when stirred (reference video).