Butter a small pyrex or 8x8 baking dish and set aside.

In a medium saucepan (preferably non-stick), combine the milk and sugar, and bring to a boil. Continue to boil, stirring constantly, for three minutes.

Remove the pan from the heat and add the peanut butter and marshmallow fluff. Stir until well combined.

Spread the fudge into the baking dish. Allow to cool to room temperature, and then refrigerate