Spray or grease a 11x13" baking pan
In a large bowl, measure out 8 cups of rice krispies cereal
Spray or grease a large pot and set aside. This will help the rice krispie treats from sticking to the sides of the pot.
On low heat, add butter to the pot and melt it.
Add the marshmallow fluff. Use two spoons to get the mixture off of the spoon.
Melt the marshmallow fluff until it is runny and melted. (3-4 minutes).
Add the vanilla.
Add the rice krispies cereal. Stir until all the marshmallow fluff covers the cereal.
Quickly scoop the mixture out into your greased baking dish.
Pat the mixture down gently into the pan.
Let it set for 30 minutes to set.