Combine brown sugar, butter, corn syrup, and salt in a saucepan over medium-low heat. Cook, stirring continuously, until mixture begins to bubble, but not boil, about 10 minutes. Remove from heat; stir in baking soda carefully. Pour over popcorn; mix until evenly coated.
Spread coated popcorn on 2 jelly roll pans.
Bake in the preheated oven, stirring every 15 minutes, until a cooled piece of popcorn is crunchy, about 1 hour.