Directions

1. Place popcorn in a large shallow roasting pan in one layer, or use two 9-by-13-inch pans; set aside.
2. Melt the butter in a large heavy saucepan over low heat. Stir in the brown sugar, salt, and corn syrup. Bring to a boil, stirring constantly. Insert a candy thermometer into pan, without the tip touching the bottom of pan. Boil mixture slowly, without stirring, until the temperature reaches the soft crack stage, about 285 degrees F. This could take at least 45 minutes, but watch thermometer temperature carefully. Meanwhile, pre-heat oven to 250 degrees F.
3. When the temperature of the mixture is reached, immediately remove from heat and stir in the soda and vanilla. This will cause the mixture to briefly foam up.