Prep. Preheat the oven to 250°F, and line a 13 x 9-inch baking dish with parchment paper. Place the popped popcorn into the prepared dish, spreading it evenly.
Make the caramel. Combine the brown sugar, corn syrup, butter, and salt in a large saucepan. Then, bring the mixture to a low boil over medium heat, stirring constantly. Allow it to cook for 3-4 minutes, stirring until slightly thickening. Be careful not to let the mixture boil over or burn. Once thickened, remove from heat and stir in the baking soda and cinnamon until fully combined.
Pour. Pour the caramel mixture over the popcorn and stir until the popcorn is evenly coated. The mixture may be sticky, but it will become easier to stir as it bakes.
Bake. Bake the caramel corn for 1 hour, stirring every 20 minutes to make sure it has an even coating. After baking, remove the parchment paper with the caramel corn from the baking dish and let it cool completely on the counter before storing.