In a saucepan over high heat, combine the butter, sugar, and evaporated milk. Bring to a boil, whisking frequently.

Reduce heat to medium and simmer for 5 minutes, continuing to whisk so nothing burns.

Remove from heat and whisk in the peanut butter until fully melted and smooth.

Whisk in the marshmallow fluff and vanilla extract until fully incorporated.

Pour into a 9×9 inch baking dish lined with aluminum foil and sprayed with cooking spray.

Refrigerate for several hours until firm, then slice and serve.