Pour cereal into an extra large bowl. Grease a 9x13'' pan lightly with butter or cooking spray. Set aside.
Add butter and 12 cups of marshmallows to a large saucepan over medium heat. Stir constantly until the marshmallows have melted--don't over cook! Remove from heat as soon as the marshmallows have melted. Stir in vanilla.
Gradually pour mixture over the cereal, stirring the cereal as it's poured to even coat everything. Stir in remaining 2 cups of marshmallows.
At this point you could add other mix-ins like chocolate pieces, or m&m's (allow the rice krispies to cool for a minute or two before adding them, so they don't melt immediately).
Pour mixture into prepared pan and press the cereal down evenly. Allow to cool for a few minutes before cutting into squares.
For best results, store rice krispies wrapped individually in plastic wrap for up to 2 days, or freeze (instructions for freezing are in the post above).
To make a smaller batch of rice krispie treats cut this recipe in half and use an 8 or 9 inch square pan.