In a heavy bottom pan on low heat melt the butter with the vanilla extract. Once melted add the marshmallows on low/medium heat and mix until gooey and melty.Now add the sugar free cereal and mix through until all the cereal is coated with the melted marshmallow mixture.
Transfer cereal mixture to an 8" x 8" baking pan lined with parchment paper. Then spread the cereal mixture evenly into the pan and use another parchment paper on top to press down to level the crispy treats.Remove top layer of parchment paper and allow to cool.