Grease 2 cookie sheets with butter, oil or pan spray. Evenly spread popped popcorn on the sheets. Preheat oven to 250'F.
In a medium sauce pan over medium heat, melt butter, brown sugar, and corn syrup together. Stir regularly to avoid burning. Bring to a boil.
Boil for 5 minutes.
Remove from heat and add the vanilla, salt, and baking soda.
Evenly pour caramel sauce over popped popcorn. Carefully toss to coat.
Bake at 250'F for about 40 minutes STIRRING EVERY 10-15 minutes.
Remove from oven and let cool until caramel has hardened. Once hardened, break into pieces.
Store in an airtight container.