Heat the oven to 250 degrees.
Layer the popped popcorn on a baking sheet lined with parchment paper, being sure to remove any unpopped kernels.
Place the popcorn in the oven to stay warm.
Add the butter, brown sugar and corn syrup to a pot over low heat until the sugar has melted.
Raise the heat to medium, and simmer for 4 minutes, or until the mixture reaches 255 degrees.
Remove the mixture from the heat, and stir in the baking soda, salt and vanilla until well combined.
Drizzle the caramel over the popcorn, and return to the oven for 45 minutes. Stir the popcorn every 15 minutes of cooking time.
Let cool before serving.