Preheat the oven to 250ºF. Grease two baking sheets with cooking spray.
Add the vegetable oil to a large stockpot set over medium heat.
Add the popcorn to the pot and cover it. Once you hear the first kernel pop, remove the pot from the heat for one minute. Return the covered pot to the heat, shaking it over the flame until all of the kernels have popped. Discard any unpopped kernels.
Melt the butter in a medium saucepan set over medium heat. Stir in the brown sugar, corn syrup and salt. Bring the mixture to a boil, stirring constantly, then allow the mixture to boil, undisturbed, for 4 minutes. Remove the caramel from the heat and stir in the baking soda.
Immediately pour the caramel over the popcorn, stirring to coat the kernels.
Transfer the coated popcorn onto the baking sheets, spreading it out into an even layer.
Bake the popcorn for one hour, stirring every 15 minutes.
Remove the caramel corn from the oven and stir it one more time before allowing it to cool completely. Break it into pieces and serve.
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