Remove any un-popped kernels.
Place popcorn in a large bowl.
Preheat oven to 300 degrees.
Melt butter on the stove top.
Add brown sugar and corn syrup.
Stir until mixture comes to a boil.
Once the mixture is boiling stop stirring and let boil for 5 minutes.
After 5 minutes remove from heat and mix in vanilla and baking soda.
Pour mixture over popped popcorn.
The easiest way to do this is to have someone mix while someone pours.
Thoroughly mix candy mixture and popcorn until popcorn is coated.
Place in a buttered baking dish.
Bake in the oven for 10 minutes.
Remove from the oven, stir and bake for another 5 minutes.
Place on buttered foil to cool.