Spray a 13 x 9-inch baking pan with cooking spray.

Melt butter in large saucepan over medium heat. Add marshmallows. Cook, stirring, until marshmallows are melted, about 4 minutes. Remove from heat and quickly stir in vanilla, then cereal and berries.

Scrape mixture into prepared pan and, using wet hands (or a rubber scraper coated with cooking spray,) quickly press evenly into pan until firm.

Serve immediately or refrigerate until serving (or pack in lunchbox with cold pack.) Consume within one day to avoid treats losing their crunch factor.