1) Pop popcorn in an air popper. You can also pop it on the stovetop if you prefer that method.
2) For sauce, add butter to a small pot over low heat. Melt and continue to let cook until the butter foams, then clears, and you notice tiny brown bits on the bottom. It should smell nutty. Remove it from the heat immediately so it doesn’t burn. It should take 5-6 minutes for this process to happen, but the final time depends on pot and temperature.
3) Vigorously stir in brown sugar and sriracha and drizzle popcorn with sauce. The sauce might separate a bit if you let it sit so stir it well before drizzling. Also season popcorn with salt if needed.