Mix brown sugar, corn syrup, butter and salt in a heavy 2-quart saucepan. Stirring constantly, bring to a boil over medium heat.
Boil 5 minutes without stirring. Remove from heat. Stir in baking soda; mix well.
Pour syrup over warm popcorn, stirring to coat evenly.
Bake for 45 minutes, stirring occasionally. Remove from oven and spread on foil that has been sprayed with cooking spray.
Cool; break apart. Store in tightly covered container.