You will need:
2 quarts popped popcorn
1 cup packed brown sugar
1/3 cup water
1/3 cup dark corn syrup (or light corn syrup)
1/4 cup butter
1/2 teaspoon salt
1 teaspoon vanilla extract
Place popped popcorn in a large bowl; set aside.
In a heavy saucepan, combine brown sugar, water, corn syrup, butter and salt. Bring to a boil over medium heat, stirring constantly.
Continue cooking, without stirring, until a candy thermometer reads 270 degrees F (132 degrees C)
Remove from the heat; stir in vanilla.
Pour over popcorn; stir until evenly coated. When cool enough to handle, quickly shape into balls.
I poured the caramel over the popcorn while it was still in the bowl. I stirred with two large spoons.
Then I dumped the bowl onto a cookie sheet and spread the popcorn out. You do have to do this very quickly.