This old-time confection dates back to the mid-1800s and remains a favorite Halloween treat. The balls are easy to make; the only trick is to watch the caramel carefully: It can go from golden brown to nearly black in the matter just of a few seconds.
Grease a large mixing bowl with butter or cooking spray.
In a 3- to 4-quart heavy saucepan, stir together sugar, corn syrup, vinegar, and salt. Bring to a boil over medium-high heat and stir in butter. Reduce heat to medium-low and boil gently until light golden brown, about 3 minutes, stirring often, then remove from heat and stir in vanilla.
Put popped corn into the large mixing bowl. Pour syrup mixture over the popcorn and stir with a spatula until popcorn is evenly coated. As soon as the mixture is cool enough to handle, butter your hands and shape the popcorn into 2- to 3-inch balls. When the balls are cool, wrap them in plastic or waxed paper.