Coat a 9x9 baking dish with cooking spray.
Sift through your popped popcorn to make sure you've taken out all the un-popped kernels. You don't want someone to break a tooth on your dessert.
Place your butter in a very large microwave bowl. Microwave for about 30-45 seconds until melted.
Add your marshmallows to the butter bowl. Stir to coat with the butter. Microwave for 1 1/2 minutes, stirring after 45 seconds. Stir to make sure the marshmallows are totally melted. Add your popcorn and stir carefully. (Really, stir carefully, or your counter will be a mess!) Stir in your chocolate chips and immediately add to your baking dish.
Spray your hands with cooking spray. Press the popcorn mixture into the pan, compacting it as much as you can. Let cool completely before slicing. You can refrigerate them to speed up this process.