Preheat oven to 250 degrees F. Place popcorn in a large bowl and set aside.
In a medium saucepan over medium heat, melt butter. Stir in brown sugar, corn syrup, and salt. Bring to a boil, stirring constantly. Boil without stirring for 4 minutes. Remove from heat and stir in baking soda and vanilla extract. Pour caramel in a thin stream over popcorn, stirring to coat. Gently stir until all of the popcorn is covered.
Place popcorn on two large shallow baking sheets and bake in preheated oven, stirring every 15 minutes, for about an hour. Remove popcorn from oven and let cool completely.
Break popcorn into pieces and mix in chopped Twix candy bars. Drizzle popcorn with melted chocolate. Let the popcorn sit until chocolate hardens, about 30 minutes. Store at room temperature in an airtight container.