Pop popcorn, let the popcorn cool, and separate unpopped kernels from popped corn.
Place popcorn in a large bowl - you may need two!
Melt butter in a large sauce pan over medium heat to start your simple caramel sauce.
After it is melted, add in the marshmallows, and brown sugar in the microwave until melted. ( you could also melt it all together in a large pot or large skillet on your stove top on medium-high heat)
Make sure all the ingredients are mixed together well- you can use a silicone spatula or wooden spoon for the best results.
Pour the butter mixture over popcorn.
Toss to coat all the popcorn.
Let sit for about 20 minutes. You can spread it out over parchment paper on a cookie sheet. If you don't have parchment paper you can spray cooking spray on the large baking sheet instead.