In a large pot, melt the butter on medium high heat. Let the butter brown and take on a nutty color.
Add the sweetened condensed milk and stir until incorporated and bubbling.
Add the marshmallows and vanilla. Stir frequently until the marshmallows are melted, about 3-4 minutes. Turn off the heat when the marshmallows are melted. Add the cereal, combine halfway with the hot mixture. Add the marshmallow fluff and fold it gently, leaving ribbons of fluff.
Pour the mixture into a greased 9x13 baking pan. Spread it until even.Let cool to room temperature before cutting into squares and enjoying.
