Prepare a 9 x 13 pan by spraying it liberally with non-stick cooking spray (I prefer Bak-kleen). In a large stockpot melt butter and marshmallows over medium heat until marshmallows are fully melted. Remove from heat and stir in vanilla and cereal until well combined. Carefully pour into prepared 9 x 13 pan. Spray hands with cooking spray and press into pan and set aside for minimum of two hours. Cut into desired squares and enjoy!