Instructions

1. Heat the oven to 250°F. Line a rimmed baking sheet with parchment paper or a silicone baking mat.
2. Place 12 cups plain popcorn in a large stockpot, roasting pan, or bowl — something large enough where you can stir it easily without popcorn falling out.
3. Melt 6 tablespoons unsalted butter in a small, heavy-bottomed saucepan (at least 1 1/2 quarts) over medium heat. Stir in 1/2 cup granulated sugar until moistened. Increase the heat to medium-high and bring to a boil. Continue to boil for 3 minutes, stirring and scraping the bottom and sides of the saucepan as needed with a rubber spatula (the mixture and any foam on top should darken to the color of light maple syrup).
4. Remove the saucepan from the heat. Carefully whisk in 1/4 teaspoon fine salt and 1/4 teaspoon baking soda (it will bubble up violently!). Continue whisking until you have a thick, glossy sauce. Immediately drizzle the mixture over the popcorn while continually stirring with the rubber spatula. Sprinkle 3 tablespoons furikake over the popcorn and stir to coat the popcorn.
5. Transfer the popcorn to the baking sheet and spread into an even layer. Bake, stirring halfway through, until a cooled piece of popcorn is very crunchy, about 30 minutes total. (To test for doneness, take a few pieces of popcorn out of the oven and let cool for 30 seconds. If they're crunchy, then the popcorn is done.) Let the furikake popcorn cool for at least 5 minutes before serving — it will crisp as it cools.