Adapted from tasteofhome.com
Place popcorn in a large bowl and set aside. In a large saucepan, combine the brown sugar, butter, corn syrup and salt; bring to a boil over medium heat, stirring constantly. Boil for 5 minutes, stirring occasionally. Remove from the heat. Stir in vanilla and baking soda; mix well. Pour over popcorn and stir until well-coated. Pour into two greased 13-in. x 9-in. baking pans. Bake, uncovered, at 250° for 45 minutes, stirring every 15 minutes. Cool completely. Store in airtight containers. Yield: about 5-1/2 quarts.