Preheat the oven to 250°F. Spray a large bowl with non stick spray. Place popcorn in the bowl and set aside.
Melt the brown sugar, butter and karo syrup in a large heavy bottom pan. Add the salt and stir until combined. Stir continually until melted.
Bring the brown sugar mixture to a light boil. Once it is at a light boil remove the spoon and continue to cook for 4 minutes without stirring or until the caramel color deepens or reaches 240°F. Do not walk away, you don't want it to burn.
Immediately remove from the heat and stir in the vanilla and baking soda. Pour over popcorn in large bowl. Add another cup or two if popcorn if needed. Stir until well coated.
Line a baking sheet with parchment paper and dump the caramel corn on top. Spread out evenly and then baking for 30 minutes, stirring occasionally. Remove from oven and add flake sea salt if desired.
Storage: Once cooled, store in a sealed container for up to one week. Cleaning pan: If the caramel is stuck in your pan, add hot water to it and bring it back to a boil. This will break down the sugars. Then use hot pads to dump and clean once cooled.