Grease a 13"x 18" baking sheet liberally; set aside.
In a large pot over medium heat, melt butter. Add sugar, sweetened condensed milk, and vanilla. Stir constantly for 5 1/2 minutes.
Transfer pot to sink. Add marshmallow fluff and chocolate chips. Stir until well combined.
Pour fudge into prepared baking dish. Shake to flatten. Top with chopped nuts, candy, sprinkles, etc. if desired. Let sit to cool.
Recipe Notes
You can use a cookie sheet with a lip or a 9" x 13" baking dish for this fudge as well. The fudge will be a different thickness and may need to sit longer.