Coat a 9x9-inch baking dish with cooking spray and set aside.
Inspect the popped popcorn and pick out any unpopped kernels.
Place the butter in a very large microwave-safe bowl and microwave 30–45 seconds until melted.
Add the marshmallows to the melted butter; stir to coat, then microwave 1½ minutes total, stirring after 45 seconds, until marshmallows are fully melted and smooth.
Carefully add the popped popcorn to the melted marshmallow mixture and gently stir to combine.
Stir in the mini chocolate chips until evenly distributed, then immediately transfer the mixture to the prepared pan.
Spray your hands with cooking spray and firmly press the mixture into the pan to compact it; let cool completely before slicing (refrigerate to speed cooling if desired).