Preheat oven to 200°F. Line two large, rimmed baking sheets with parchment paper. Set aside.
Lightly spray a very large mixing bowl with nonstick cooking spray. Add the popped popcorn. Set aside.
Measure out the vanilla and baking soda and set aside.
In a large saucepan, add the brown sugar, butter, corn syrup, and salt. Bring to a boil over medium-high heat, stirring constantly with a silicone spatula or wooden spoon. As soon as the mixture is at a steady boil, set a timer for 5 minutes. Continue to boil, stirring constantly.
When the timer goes off, remove the pan from the heat and immediately add the vanilla and baking soda and stir vigorously to combine. The mixture will bubble up and hiss when you add these ingredients, so stand back a bit.
Immediately pour the hot caramel over the popcorn and carefully stir to coat the popcorn with the caramel. Once the popcorn is well coated, divide it between the two prepared baking sheets, spreading into an even layer. Some of the popcorn will clump together - that’s ok. Just spread it out as best you can.
Bake in the preheated oven for 45 minutes, stirring every 15 minutes.
Let the caramel corn cool on the baking sheets, breaking apart any large clumps as it cools.
Store in an airtight container at room temperature for up to a week.
