Line a baking sheet with parchment paper and preheat oven to 200 degrees F.

Mix brown sugar, butter, corn syrup, and salt in a heavy 2-quart saucepan. Stirring constantly with a heat-resistant silicone spatula, bring to a boil over medium heat.

Remove from heat, then stir in vanilla and baking soda. Mix well; the caramel will become foamy and fluffy looking.

Pour syrup over warm popcorn in a large mixing bowl, gently folding popcorn and caramel with a silicone spatula to coat evenly. Quickly pour popcorn onto baking sheet lined with parchment paper and spread across the sheet to create a thin layer, as uniform as you can get it.

Bake for 45 minutes, stirring the popcorn gently at the 15 and 30 minute marks.

Let cool to crisp up the caramel, then break apart. Store in an air-tight container for up to 4 days.