Preheat the oven to 350°F/180°C. In a large pot with a lid, heat the oil and popcorn kernels over medium heat, shaking the pot occasionally, until the kernels pop. Remove from the heat when the popping sounds slow down to one every 2 or 3 seconds so you don’t burn the popcorn. Transfer the popcorn to a large metal bowl or a deep, oven- proof casserole dish, and remove any remaining unpopped kernels or burned bits; set aside. If any of the popcorn burns, the acrid flavor will permeate the whole batch; it’s better to have
too many unpopped kernels (which can be reused) than to risk burning even one piece of popcorn.
In a medium-large saucepan, combine the sugar, water, and corn syrup. Use a wet paper towel to wipe down the sides of the pot and have a cup of water with a pastry brush nearby. Bring the mixture to a vigorous boil over high heat. If sugar crystals form on the sides of the pan, brush them down with the wet pastry brush. When the sugar syrup begins to take on a golden hue along the edges, reduce the heat to medium-low so it doesn’t cook too quickly, and cook, shaking the pot frequently to distribute the cooked sugar, until the caramel has a dark golden color, then immediately remove from the heat. Add the butter, baking soda, and salt, standing back from the saucepan to avoid the steam. Stir in the butter until it is evenly distributed and the caramel has taken on an orange hue.
Pour the caramel over the popcorn and stir it with a rubber spatula. When the caramel becomes too stiff to work with, put the entire bowl in
the oven and let it heat for 3 minutes to soften
the caramel, then remove and continue to stir. Repeat this process until all the popcorn is at least partially coated with caramel, scraping the bottom of the bowl each time to make sure you’re using any caramel that has pooled there.
Line a sheet pan with parchment paper, pour the caramel-coated popcorn onto the pan, and use the rubber spatula to spread and press the popcorn into one layer. Set the popcorn aside to cool.
When the popcorn has cooled to room temperature, temper the chocolate using any tempering technique, then bring it up to working temperature. When the chocolate is ready, divide it between two pastry bags. Cut a small hole in the end of one bag and drizzle the chocolate across the top of the popcorn in a zigzag pattern. When you reach the end of
the tray, repeat the zigzag pattern in the opposite direction so that you create an uneven crosshatch pattern; you should use all the chocolate in the first bag. Set the pan in a cool place until the chocolate has set enough that it is firm to the touch (to speed up this process, you can put the tray in the refrigerator for a minute). Flip the entire slab of popcorn so that the chocolate- covered side is down, and drizzle the other side of the popcorn with the second bag of chocolate. (If the chocolate stiffens up too much while the first side is setting, it can be heated for 5 to 10 seconds in the microwave or over a saucepan of hot water to bring it back to working temperature.) Set the popcorn aside to let the chocolate solidify. When the chocolate has set, break the popcorn into large chunks.