I'm in love with this recipe - it's perfect when I have some marshmallows left and they aren't fresh enough for s'mores. Typically I add in chocolate chips or pecans at the end (or both) and a bit of salt.
8 cups popcorn or 1 bag popcorn
1/2 cup butter
1 cup brown sugar
1 tablespoon light corn syrup
20 large marshmallows
Pop the popcorn and place it in a bowl - make sure that there are no un-popped kernels there.
Melt the butter in a medium pot on the oven. Add brown sugar and corn syrup and stir until well blended. Bring the mixture to a boil and boil for about 2 minutes. Add marshmallows and stir continuously on low heat until all melted.