Pop the microwave popcorn according to instructions and pour into a large mixing bowl. Combine condensed milk, brown sugar, corn syrup and butter in a saucepan. Mix together well. Cook over low heat until warm and of a syrup-like consistency. When ready, pour over popcorn. Stir until popcorn is well coated then shape into balls being careful not to burn hands. This will make a lot of caramel corn, so be sure to adjust the recipe as you see fit!
Tip: Coat spoon and hands with butter or nonstick cooking spray to prevent popcorn from sticking.