Over medium low heat, we melt the butter in our dutch oven. We then stir in the brown sugar. Finally, we add the marshmallows and stir until they dissolve completely. We remove the pot from the heat.
We add all of the popcorn to the marshmallow mixture and stir with a wooden spoon until the popcorn is well coated. The mixture is somewhat difficult to stir due to its thickness and our desire to keep the popcorn in tact, so we do the best that we can. Just before the popcorn is well-coated, we add the M&M’s, distributing them evenly so that little additional stirring is required to mix them well (to prevent breakage).
Using generously buttered hands, we fashion balls of the popcorn-marshmallow mixture. Each ball we make about 3 inches in diameter and place on a large piece of wax paper on the counter. When we are done, we have nine popcorn balls. After we wash up, we place each popcorn ball on a serving plate and cover with plastic wrap for serving later.