Preheat your oven to 250°F. Line a large baking sheet with parchment paper or a silicone baking mat.

Pop the popcorn kernels by heating vegetable oil in a large pot over medium-high heat. Add the kernels, cover with a lid, and shake occasionally until popping slows to about 2 seconds between pops. Transfer the popped corn to a large mixing bowl, removing any unpopped kernels.

In a medium saucepan, combine butter, brown sugar, corn syrup, and salt. Cook over medium heat, stirring constantly until the butter melts and sugar dissolves. Once it bubbles, stop stirring and let it boil for exactly 4 minutes.

Remove from heat and stir in baking soda and vanilla extract. The mixture will foam up.

Pour the hot caramel over the popped corn and toss to coat every piece quickly.

Spread the caramelized popcorn in an even layer on the prepared baking sheet. Bake for 45 minutes, stirring every 15 minutes.

Remove from the oven and let cool completely on the baking sheet. Break apart any large clusters and transfer to a serving bowl.