Preheat oven to 250°F.
Melt butter, brown sugar, corn syrup and salt in a saucepan over medium heat. Bring to a boil and let simmer 5 minutes without stirring.
Remove from heat and stir in vanilla and baking soda.
Pour over popped corn (and optional mix-ins) and mix well to combine.
Spread mixture over two 9x13 pans and bake 1 hour, stirring every 15 minutes.
