Prep: Line a large 9 x 12 inch rimmed baking sheet or pan with parchment paper.
Melt: Bring a medium saucepan of water to a boil. Place dark chocolate in a heatproof bowl and set just over the boiling water (the bottom of the bowl should not touch the water). Stir chocolate until most of it has melted. Remove from heat and continue stirring until all pieces are melted. In a different bowl, repeat melting process with the white chocolate. Stir half of the popcorn into the white chocolate.
Pour: Pour the dark chocolate into the prepared baking sheet, spreading evenly over the sheet. Drizzle white chocolate mixture on top. Use a knife to create swirls in the chocolates. Sprinkle evenly with remaining popcorn and sea salt.
Set: Set in the refrigerator to chill until hard, at least 30 minutes. Break bark into pieces either by hand or with a knife. Store leftovers in an airtight container in the refrigerator.