Pop popcorn. Put into a large bowl.
On the stove, melt the butter and marshmallows together. Really low heat, stirring often.
When melted, add the Jell-o. Mix it in.
Pour over popcorn. Mix it in. Really quickly.
Let it cool just a bit - it will be very hot! Spray a cookie sheet with baking spray.
Form the popcorn mixture into balls. I like to put butter or cooking spray on my hands so it doesn't stick too much. Put on cookie sheets.
Wait til completely cooled and set (30 minutes), then wrap with Saran Wrap.