Pop your popcorn, then set it aside while you make your marshmallow topping.
16 cups popped popcorn
In a heavy saucepan over medium low heat, begin to melt your butter. As the butter melts, add the marshmallows and brown sugar.
1/2 cup butter, 6 ounces marshmallows, 1/2 cup brown sugar
Stir constantly until the marshmallows have melted and are completely incorporated into the mixture. Remove from the heat and add the vanilla, stirring to mix thoroughly.
1 teaspoon vanilla bean paste
Place your popcorn and marshmallow mixture as near each other as possible. Using a curved spatula pick up batches of marshmallow from your pot and drizzle them over the popcorn. Once the marshmallow has been added, rub the palms of your hands lightly in butter to keep them from sticking and carefully mix up the popcorn to distribute the marshmallow while not breaking the popcorn.
Serve immediately. This will keep in a sealed container for 2-3 days after making it.