Into large bowl, measure cereal. Spray 13X9 inch pan with non-stick cooking spray. In large microwavable bowl, microwave 5 cups marshmallows, the chocolate chips, corn syrup and butter uncovered on high for 2 minutes to 3 minutes 30 seconds, stirring after every minute, until melted and smooth when stirred. Stir in vanilla.
Pour over cereal; quickly toss until completely coated. Stir in 2 cups of mini marshmallows.
Press mixture evenly in pan, using buttered back of spoon. Let stand uncovred at least 1 hour, or refrigerate if you prefer a firmer bar. Store loosely covered at room temperature up to 2 days.
*HINT* If you spray your bowl with cooking spray before using it, you'll have less mess to clean when you're done!