In a saucepan, combine 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 onto two greased 13-in x 9-in x 2 in baking pans (or close to that). Bake uncovered at 250 degrees F for 45 minutes stirring (and turning over) every 15 minutes. Cool completely. Store in airtight containers or plastic bags. ENJOY.