Prepare the popcorn by popping the kernels using your preferred method (stovetop, microwave, or popcorn machine). Set aside in a large bowl.

Preheat your oven to 250°F (120°C) and line a baking sheet with parchment paper.

In a medium-sized saucepan, combine the sugar, butter, corn syrup, and salt. Place it over medium heat and stir until the butter melts and the mixture comes to a gentle boil.

Allow the mixture to boil, without stirring, for 4-5 minutes until it reaches a rich golden brown color. Remove from heat.

Carefully stir in the baking soda and vanilla extract. The mixture will bubble vigorously, so be cautious.

Pour the hot toffee mixture over the popcorn in the large bowl. Use a spatula or wooden spoon to gently toss and coat the popcorn evenly.

Transfer the coated popcorn to the prepared baking sheet, spreading it out in a single layer.

Bake in the preheated oven for 45 minutes, stirring every 15 minutes to ensure even baking.

Remove the baking sheet from the oven and let the popcorn cool completely. As it cools, the toffee coating will harden, creating a delightful crunch.

Once cooled, break apart any large clumps and transfer the Butter Toffee Popcorn to an airtight container for storage.