Melt your butter, vanilla extract, and marshmallows in a saucepan over medium-low heat. Once the marshmallows are melted, remove from heat and stir in the vanilla. Quickly stir in the cereal. Then stir in your candy corn and M&M's.
Pour the marshmallow-cereal mixture into your prepared baking dish. Press to flatten. (I like to spray my hands with cooking spray and use them to press the treats into the pan). Top with your halloween sprinkles.