Prepare a baking sheet with non-stick cooking spray. In a large stock pot, melt marshmallows and butter over medium-low heat. Once fully melted, whisk in the pudding mix until fully incorporated. Fold in the popcorn and graham cracker cereal until coated with the marshmallow mixture. Spray an ice cream scoop with non-stick spray and portion out the popcorn mixture and gently form into a ball. Place formed popcorn balls into the prepared baking sheet. Continue to form popcorn balls with the remaining mixture. Allow to set in the fridge for 30 minutes before serving. Popcorn balls can be stored in the fridge to stiffen or stored in a zip to bag onthe cournter for a softer consistency.