Melt butter; stir in brown sugar, corn syrup, and salt. Bring to boil, stirring constantly. Boil, without stirring, 5 minutes.
Remove from heat; stir in baking soda and vanilla.
Put popped corn in large roaster; gradually pour syrup over popcorn and mix well.
Bake at 250Â° for 1 hour, stirring every 15 minutes.
Remove from oven and put on waxed paper to cool completely.
Break apart and store in tightly covered container.
NOTE: If adding nuts, spread nuts, one layer deep on top of popped corn then cover the nuts and popcorn first with the cooked caramel. This makes them sticky and keeps them from going to the bottom of the pan when mixing.