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 extract. Gradually pour over
popped corn, mixing well. Turn into 2 large shallow baking pans. Bake at 250 degrees
F for 1 hour, stirring every 15 minutes.