Preheat oven to 120 degrees C. Place popcorn in a very large bowl.
In a medium saucepan over medium heat, melt butter. Stir in brown sugar, golden syrup and salt. Bring to the boil, stirring constantly. Boil without stirring 4 minutes. Remove from heat and stir in bicarb and vanilla. Pour in a thin stream over popcorn, stirring to coat.