Grease a glass plate or baking dish with a small amount of butter.
Put peanut butter and marshmallow fluff in a large bowl and set aside.
Melt butter over medium heat in a large saucepan.
Add sugar and milk and bring to a rolling boil over medium-high heat for about 4 minutes.
Remove from heat and add vanilla.
Pour mixture over peanut butter and marshmallow fluff and stir well until mixed thoroughly.
Pour into greased dish and let set for 1 hour. Refrigerate if you need it to set faster.
Cut into squares or eat it with a spoon if it's raining outside. :)