This is a hit - one of my favorites from allrecipes.com - it is rated 5 out of 5 by 745 members. I made it again yesterday!
1 cup butter
2 cups brown sugar (light or dark, I have tried both)
1/2 cup corn syrup
1 teaspoon salt
1/2 teaspoon baking soda
1 teaspoon vanilla extract
5 qts. popped popcorn
1. Preheat oven to 250 degrees. Place popcorn in very large bowl.
2. In a medium saucepan over medium heat, melt butter. Stir in brown sugar, corn syrup and salt. Bring to a boil, stirring constantly. Boil without stirring 4 minutes. Remove from heat and stir in soda and vanilla. Pour in a thin stream over popcorn, stirring to coat.
3. Place in two large shallow baking dishes (I used large cooking sheets) and bake in preheated oven stirring every 15 minutes for 1 hour (I have never waited an hour - for me it's like 15 minutes at 300). Remove from oven and let cool completely before breaking into pieces.