Pop popcorn and place it in a large bowl, and preheat the oven to 250 degrees.

Melt butter in a medium saucepan. Stir over medium heat and mix in salt, brown sugar, and corn syrup. Stir constantly while bringing to a boil

Boil without stirring for 4 minutes and then remove from heat. Mix in vanilla and soda.

Pour slowly over popcorn, stirring to coat.

Place in 2 shallow baking dishes and pop in the oven to bake for 1 hour. Stir every 15 minutes. Let cool completely before breaking up the popcorn and eating!