Melt marshmallow in the microwave in 30-second intervals until smooth (about 2 minutes). Stir.
Add butter and microwave another 30 seconds. Stir.
Pour marshmallow/butter mixture over popped popcorn. Stir to coat evenly.
Rub a small amount of butter on your hands to prevent the popcorn from sticking and roll out into oval shaped balls. Place on baking sheet covered in parchment or waxed paper.
Makes 18 popcorn balls.
Notes
Note: You can add mini dinosaur toys and mold the popcorn around them for a fun surprise! You can also add sprinkles or tint the marshmallows with food coloring before pouring them over the popcorn for colored dinosaur egg popcorn balls.