Start by popping your popcorn. You can use an air popper or stovetop method. Once popped, place the popcorn in a large mixing bowl and sprinkle with kosher salt. Set aside.
In a medium saucepan, melt the butter over medium heat. Once melted, add the dark brown sugar and corn syrup. Stir continuously until the mixture comes to a boil. Let it boil without stirring for about 4-5 minutes.
Remove the saucepan from the heat and carefully stir in the vanilla extract and baking soda. Be cautious, as the mixture will bubble up. Stir until well combined.
Immediately pour the caramel sauce over the popped popcorn. Use a spatula to gently fold the popcorn and caramel together until all the popcorn is evenly coated.
Preheat your oven to 250°F (120°C) and line a baking sheet with parchment paper. Spread the caramel-coated popcorn evenly on the baking sheet. Bake for 1 hour, stirring every 15 minutes to ensure even coating and to prevent burning.
Once baked, remove the caramel popcorn from the oven and let it cool completely on the baking sheet. Once cooled, break it into pieces and enjoy your Homemade Caramel Popcorn!