In a microwave safe bowl, melt marshmallows and butter for 1 minutes.

Remove from microwave and pour in vanilla. Stir until the mixture is completely smooth.

Add 6 cups of rice crispies and stir until the cereal is well coated.

Line a 9x13 baking pan with parchment paper and pour the cereal mixture into the pan. Place a piece of parchment paper over the top and press the crispies down into an even layer.

Let sit on the counter for an hour or in the fridge for 30 minutes until set.

Lift the treats from the pan and slice into 12 pieces.