Grease a 9 x 13 dish with butter.

In a large sauce pan, melt the butter over medium low heat.

Add in all but 2 cups of marshmallows. Stir until marshmallows are melted and combined well with the butter.

Add in the vanilla and stir to combine.

Remove from the heat and add in the rice krispies and stir until cereal is well coated. Add in the remaining 2 cups of marshmallows and stir. Don't oven mix, you want the marshmallows to stay as whole as possible.

Add to the prepared 9 x 13 pan. Use a piece of waxed paper or parchment paper to press the cereal into pan until level.

Allow to cool. Once cooled, cut into squares.