Make popcorn according to the instructions on the bag.
In a medium saucepan over medium heat, melt the butter. Add the marshmallows stirring with a wooden spoon until melted. Add the popcorn mixing rapidly from bottom to top until well coated. Add the candy coated chocolate, mixing well to distribute evenly.
Butter a 9 by 9-inch baking dish with butter and line with plastic wrap. Pour the popcorn mixture into the prepared pan. Press down with the back of spoon and let it cool. Cut in 4 squares and serve.