1Heat oven to 250°F. In 13x9- or 15x10-inch pan, toss popcorn and cereal; set aside.
2In large saucepan, melt butter over medium heat. Stir in brown sugar, corn syrup and salt. Heat to boiling; boil 5 minutes without stirring. Remove from heat; stir in baking soda and vanilla (be careful; mixture will foam). Pour over popcorn mixture; stir until well coated.
3Bake uncovered 40 minutes, stirring every 10 minutes.
4Sprinkle marshmallows over popcorn mixture. Broil 2 to 3 minutes or until marshmallows start to turn golden brown. Sprinkle chocolate chips over top and stir (mixture will be a bit gooey). Cool completely.