Preheat the oven to 250°F. Line 1–2 large baking sheets with parchment paper or foil.
Melt the butter in a large saucepan over medium heat. Stir in the brown sugar, corn syrup, and salt. Bring to a boil, stirring frequently.
Once boiling, cook for 5 minutes without stirring. Remove from the heat and carefully stir in the baking soda and vanilla extract. The mixture will foam.
Spread the popped popcorn on the prepared baking sheets, removing any unpopped kernels. Pour the caramel mixture over the popcorn and gently toss until evenly coated.
Bake for 60 minutes, stirring every 15 minutes.
Remove from the oven and let the popcorn cool completely.
Place the white chocolate chips and dark chocolate chips in separate microwave-safe bowls. Microwave each in 30-second intervals, stirring after each interval, until smooth and melted.
Transfer each melted chocolate to a separate zip-top bag. Snip off a small corner of each bag and drizzle the chocolates evenly over the cooled popcorn.
Let the chocolate set before serving.
Notes
Store in an airtight container or bag at room temperature for up to 1 week.