• Preheat your oven to 200 degrees F.
• Set a large sheet of wax paper on an open area on your kitchen counter to set the popcorn balls once they are formed.
• Cut off 18 square sheets of wax paper just large enough to hold the 3-inch popcorn balls.
• Fill a medium bowl with cool water and set it next to the wax paper (for dipping hands while forming the balls). Or, keep an additional 2 or 3 tablespoons of butter nearby for coating hands.

Pop the Popcorn

• Pop the popcorn, transferring the batches to a large (16- to 20-quart) metal bowl or large turkey roasting pan, being sure to remove any unpopped kernels. Transfer the bowl to the oven to keep the popcorn warm in between batches and while you make the caramel syrup.

Make the Caramel Syrup

• In a heavy saucepan, add the granulated sugar, brown sugar, corn syrup, water, vinegar, and salt and place it over MEDIUM heat. Cook, stirring frequently, until bubbling. Clip a candy thermometer on the side of the pan and continue to cook, stirring frequently, until the temperature reaches 200 degrees F. At this point, stir the mixture constantly to ensure it won’t burn, and continue cooking to a temperature of 245 degrees F. This process is slow, so just pull up a chair. It can take about 20 minutes.
• As soon as the temperature reaches 245 degrees F, quickly move the pan off the heat. Immediately add the butter pats and vanilla, stirring until the butter has melted.

Mix and Form the Popcorn Balls

• Quickly remove the bowl of popcorn from the oven and Immediately pour the caramel syrup over the warm popcorn. Use 2 large spoons to stir and toss the popcorn until the caramel has been distributed throughout. As soon as the mixture is just cool enough to handle (should be within a minute or two at the most) use your hands to form it into 3- to 3 ½-inch balls, setting them down on the piece of wax paper. Work quickly (the balls can be difficult to form if the syrup cools too much). If your hands get too sticky, dip your hands in the cool water or coat with softened butter in between forming the balls.
• Once all the popcorn balls have been formed, wrap each one, individually, in a square piece of wax paper.
• Store at room temperature.