Begin by popping your air-popped popcorn according to your machine’s instructions. Aim for about 5 quarts, and be sure to set it aside in a large mixing bowl.
In a large pot over medium heat, melt the butter. Once melted, add the light brown sugar, light corn syrup, and salt. Stir the mixture continuously until it comes to a boil. Allow it to boil for about 4-5 minutes without stirring, until it reaches a deep golden color.
Remove the pot from heat and carefully stir in the baking soda and vanilla extract. The mixture will bubble up, so take care during this step.
Pour the hot caramel sauce over the popped popcorn. Use a spatula to gently fold the popcorn until it is evenly coated with the caramel.
Fold in the chopped Twix candy bars, making sure they are evenly distributed throughout the popcorn.
Preheat your oven to 250°F (120°C). Spread the caramel popcorn mixture onto a baking sheet lined with parchment paper. Bake for 45 minutes, stirring every 15 minutes.
Once the popcorn is done baking, remove it from the oven and let it cool completely. Meanwhile, melt the semi-sweet chocolate chips in a microwave-safe bowl, stirring every 30 seconds until smooth. Drizzle the melted chocolate over the cooled popcorn.
Allow the chocolate to set completely before breaking the popcorn into pieces. You can speed up the process by placing it in the refrigerator for a short while.
