Butter the inside of a large mixing bowl and pour the cereal in. Set aside.
Rub the inside of baking dish or pan with butter. Set aside.
Rub the inside of the sauce pan or pot with butter.
In medium sauce pan, add the ⅓ cup salted butter along with the marshmallows and melt over Medium-Low heat. Stir until marshmallows are completely dissolved.
Remove from heat and stir in the vanilla.
Pour the hot mixture over the cereal. Briskly stir until well coated.
Using a buttered hand, a buttered spatula or a piece of non-stick foil (non-stick side), evenly press cereal mixture into the prepared baking dish. Let Cool.
Cut into squares and serve.