Pop the popcorn and set aside in a large bowl.
Coat an 8X8 inch baking pan with PAM Cooking Spray.
Combine marshmallows and butter in a saucepan over medium heat, stirring until completely melted and smooth.
Pour the marshmallow mixture over the popcorn and stir until popcorn is well coated.
Scoop the entire mixture into the prepared pan and press down until evenly distributed. (I usually spray the back of a wooden spoon with PAM Cooking Spray and use that to press the mixture into the pan – if you don’t spray the spoon first, it will stick and make it much more difficult! Sometimes I actually just spray my fingers and use those to press the mixture down!)
I sprinkled some M&Ms on the top just for decoration.
Let cool for a few minutes and then cut into squares and serve.