Begin by air popping the popcorn and transferring it to a large bowl, making sure to remove any unpopped kernels and bad pieces.
In a saucepan over medium heat, combine the butter, brown sugar, dark corn syrup, and salt. Allow the mixture to come to a boil, which should take about five minutes, until the butter is completely melted and blended in.
Once boiling, add the baking soda to the caramel mixture and stir quickly to help it expand.
Drizzle the caramel over the popcorn and use a metal spatula to mix thoroughly, ensuring that each kernel is well coated.
Spread the caramel-coated popcorn onto a baking sheet and place it in the oven. Bake for one hour, stirring and breaking up clumps with a metal spatula every 15 minutes.
After an hour, remove the popcorn from the oven and let it cool until it becomes crunchy.
