Melt butter, stir in brown sugar, corn syrup and salt. Bring to a boil, stirring constantly; boil without stirring for 5 minutes. Remove from heat, stir in baking soda and vanilla. Gradually pour over popcorn, mixing well.
Turn into two large shallow baking or roasting pans. Bake in 250 degree oven for 1 hour, stirring every 15 minutes. Remove from oven, cool completely. Break apart and store in tightly covered container.