In a large pot over low heat, melt the butter. Stir in the vanilla and marshmallows. Continue stirring until marshmallows are just melted. Do not over-cook.
Remove from heat and add cereal. Stir until almost combined, then add sprinkles. Carefully mix them to distribute, then pour mixture into prepared pan. With buttered wax paper, press cereal into an even layer. Allow to cool before cutting.
*You may use another variety of Chex cereal, or 6 cups of crispy rice cereal.