Spray large mixing bowl with pan spray; add cereal and 8 oz marshmallows and set aside.
Heat butter, 3 lb marshmallows, chocolate chips and vanilla extract in heavy-bottom large saucepan over medium heat.
Stir constantly until melted and mixture is smooth; remove from heat and cool 5 minutes.
Pour chocolate mixture over cereal mixture, and mix well to coat; press mixture into greased full sheet pan. (Place a piece of parchment paper on top of mixture, and press with a rolling pin to assist.)
Cool at room temperature approx. 1 hour before portioning.
Check out our recipe for Golden Grahams™ S'mores Bars. A delicious recipe featuring only the best products from General Mills Convenience and Foodservice.