- Preheat oven to 250°F. Keep popcorn warm in oven while preparing syrup.
- Line two baking pans with foil; spray with cooking spray.
- Combine brown sugar, butter, and corn syrup. Cook and stir over medium heat until mixture boils. Continue boiling without stirring for five minutes.
- Remove from heat; stir in baking soda and vanilla. Pour caramel over popcorn; stirring gently to coat. Place on prepared baking pans.
- Bake for 20 minutes stirring after 10 minutes. Let cool. Store in tightly covered containers for up to one week.