Heat the butter, sugar and milk on low until butter is melted, then turn up the temperature until mixture boils, stirring regularly. Once it boils, turn heat to medium and stir constantly for five minutes.
After five minutes, remove from heat and stir in chocolate chips until combined.
Stir in vanilla and marshmallow fluff until uniform texture. Spread in a greased 9x13 baking dish and let cool at room tempurature.
Wait until completely cooled to cut, and store at room temperature.