In a heavy pan bring to boil the butter, sugar, corn syrup, and salt.Boil vigorously without stirring for 6 minutes.Remove from heat and stir in the baking soda and vanilla.Put the popcorn into 2 large roasting pans.Pour over the syrup.Bake in a 250 º F. oven for 1 hour….stir every 15 minutes.Cool and break apart.