Spray a 9x13" pan with non stick cooking spray; set aside. Spray a large mixing bowl with non stick cooking spray. Measure 1 cup of the marshmallows and set aside.

In a large glass bowl combine remaining marshmallows, chocolate chips, butter, and corn syrup. Cook in the microwave till melted, stirring every 30-60 seconds. Stir in vanilla extract. 

Pour the cereal into your greased bowl. Quickly pour the melted chocolate mixture over the cereal with the reserved 1 cup of marshmallows. Stir gently till well coated.

Press the warm mixture into your prepared pan. I find that it works best to use wet hands, but you can use a spatula or wooden spoon.

You can let the bars sit for 5 minutes and serve them still warm, or you can let them stand at room temperature for at least 30 minutes for easier cutting.