Preheat the oven to 250 degrees F and line a baking sheet with parchment paper or foil. You may need 2 baking sheets based on the size that you are using.
In a large saucepan, heat the butter until melted. Then stir in the brown sugar and corn syrup. Bring the mixture to a boil, stirring frequently. Then boil for approximately 5 minutes without stirring. Remove from heat and stir in the baking soda and vanilla extract. The mixture will foam up when the baking soda is added.
Place the popped popcorn on the baking sheets and remove any of the unpopped kernels. Pour this mixture over the popcorn and gently toss the popcorn to coat it with the mixture.
Bake the popcorn for 60 minutes, stirring the popcorn every 15 minutes.
Remove the popcorn from the oven and let the popcorn cool.
Place the white chocolate chips and dark chocolate chips in 2 separate microwave safe bowls. Heat in 30 second intervals, stirring after each interval, until the chocolate is melted.
Add the melted chocolate to 2 separate ziplock bags and cut the corner off of the bag with scissors. Drizzle both chocolates on top of the popcorn evenly.
Notes
Store in an airtight container or bag at room temperature for up to 1 week.