Preheat oven to 250 degrees F and place the popped popcorn in a large bowl, discarding any unpopped kernels. Set bowl aside.
In a medium saucepan, melt the butter over medium heat. Stir in the brown sugar, corn syrup, and salt, then stir constantly until the caramel comes to a full boil.
Stop stirring and let the caramel mixture boil for 4 minutes without stirring. Remove the caramel from the heat and stir in the baking soda and vanilla. Pour the caramel over the popcorn in the bowl and gently stir to coat the popcorn.
Divide the caramel popcorn evenly between two large baking sheets and bake at 250 degrees F for 1 hour, stirring every 15 minutes. Remove from the oven and sprinkle with the chopped TWIX candy bars. Return the popcorn to the oven for 3 more minutes to let the TWIX just begin to melt into the popcorn.
Remove the TWIX caramel corn from the oven and drizzle with melted chocolate, then cool completely until the chocolate hardens. This can be sped up by sticking the caramel popcorn in the freezer or fridge. Break up into smaller chunks and store at room temperature in an airtight container.