In a saucepan, combine sugars, corn syrup, water, vinegar, and salt. Attach a candy thermometer to the side of the pan; heat to boiling over medium-high heat, stirring all of the time. Cook until mixture reaches 260°F.
¾ cup granulated sugar, ¾ cup brown sugar, ½ cup light corn syrup, ½ cup water, 1 teaspoon white vinegar, ¼ teaspoon salt
Reduce heat to low; stir in butter until melted. Pour syrup over corn in a thin stream. Stir until well coated. Cool slightly.
¾ cup 1 ½ sticks butter
While the caramel corn is cooling, cut squares of waxed paper and spray them with cooking spray.
cooking spray
Butter your hands. Scoop a small handful of popcorn mixture and form into a 3" ball. Place on a square of waxed paper, twist and turn paper to close. *It is key to seal up the popcorn balls at this point. Do NOT let them cool before you do this or they will harden and turn into rocks.
Notes
I used 1 package of butterless microwave popcorn, and it was exactly 8 cups.Source: My 8th grade Home Economics teacherYou're Gonna Bake It After Allbakeitafterall.com