Preheat oven to 250 degrees F (95 degrees C).

Place the popped popcorn in a large oven-safe container or bowl that is large enough to allow for vigorous stirring (you can use two containers if you don't have one that is large enough. I use a large disposable metal pan (see photo above), which I wash and re-use repeatedly.

In a medium saucepan, combine butter, brown sugar, corn syrup, and salt over medium heat. Stirring, constantly, bring to a boil. Stop stirring and allow to boil for 4 minutes.

Remove from heat and quickly stir in the baking soda and vanilla. The caramel will immediately start to foam. Working quickly, pour the caramel evenly over the popcorn and stir with a rubber spatula until it is well coated and becomes harder to stir.

Bake the popcorn in the preheated oven for 45 minutes to 1 hour, stirring every 15 minutes. With every stir you will notice that the popcorn is becoming drier. Once it becomes quite dry and no longer sticks together, remove from oven and stir again to break apart remaining clusters.