Place the butter, sugar and milk into a saucepan over high heat on the stove, and bring to a boil, whisking often to combine. 

Once boiling, reduce the heat to medium and boil for five minutes. 

Remove the mixture from the heat and whisk in the chocolate chips until melted. 

Whisk in the marshmallow fluff and vanilla extract. 

Pour the mixture into a 9x9 baking dish that has been lined with aluminum foil and sprayed with cooking spray. 

Let set for several hours in the fridge before slicing to serve.