Pop the popcorn:
Spray two large heatproof rubber spatulas and a very large metal bowl (at least twice the volume of the popped popcorn) with nonstick cooking spray, or lightly wipe with vegetable oil. Heat the oil in an 8-quart or larger heavy-based stockpot over high heat. After a minute, put a popcorn kernel in the pot and cover. When the kernel pops, the oil is hot enough. Add the rest of the popcorn kernels. Cover the pot and shake it back and forth over the heat to agitate the kernels. Keep moving the pot as the popcorn pops. Remove the pot from the heat when the popping slows almost to a stop (it’s better to have a few unpopped kernels than burnt popcorn) and immediately pour the popped corn into the large metal bowl. Search through the popcorn, removing any unpopped kernels (which fall to the bottom of the bowl) or burnt pieces.
Make the caramel:
Measure the baking soda into a small dish so it’s ready to go. Line two rimmed baking sheets with parchment, foil, or nonstick silicone mats. In a 4-quart or larger saucepan, combine the sugar, salt, butter, and 1 cup water. Gently stir with a wooden or metal spoon just enough to immerse the sugar. Brush down the sides of the pot with water and a clean pastry brush. Cook the sugar mixture over high heat without stirring until it melts and bubbles and turns a very light golden caramel color on top; this will take 10 to 20 minutes, depending on your stove. The caramel will be darker than it appears on the surface, so don’t overcook. At this point, remove the pot from the heat.
Brush the sides of the pan clean with a wet pastry brush after you stir the water, sugar, salt, and butter.
Watch the boiling syrup, but don’t stir or swirl it. Once the syrup turns a golden caramel color, take it off the heat.
Working quickly off the heat, thoroughly whisk the baking soda into the caramel. Do this in or near the sink in case it spills over. The baking soda aerates the cararamel, which makes it easier to eat when it’s cool, but causes it to bubble vigorously now, so be careful. Immediately pour the bubbling caramel over the popcorn in the bowl. Only use the caramel that pours out easily; don’t scrape the sides of the pot (the sugar on the sides of the pot crystallizes easily and can cause the caramel to do the same).
Whisk in the baking soda. The caramel will bubble vigorously and quickly rise up in the pan.
Pour the bubbling caramel onto the popcorn, taking care not to get any on yourself. Don’t scrape out the pot.
Using the heatproof spatulas, toss the caramel with the popcorn. When the popcorn is thoroughly coated, pour it onto the lined baking sheets and use the spatulas to pat it into one flat layer. As soon as it’s cool enough to touch, use your hands to break the layer into smaller clusters. Let them cool completely and then store in an airtight container for up to a week.
Toss the popcorn and caramel as you would a salad, using two heatproof spatulas.
Pat the caramel popcorn into a single layer on lined baking sheets.
Spicy: Stir 3/4 tsp. cayenne into the baking soda and add to the caramel as directed.
Nutty: Toss 2 cups lightly salted peanuts with the popcorn before pouring on the hot caramel.
Photo: Scott Phillips