The hardest part of this recipe is waiting for your caramel popcorn to cool before you eat it!
Adjust oven rack to middle position and heat oven to 250 degrees. Spray inside bottom and sides of 13‑by-9‑inch metal baking pan with vegetable oil spray. Place popcorn in baking pan.
In large saucepan, melt butter over medium-high heat. Add brown sugar, corn syrup, and salt to saucepan. Bring mixture to boil. Reduce heat to medium-low and simmer, stirring occasionally with rubber spatula, until mixture thickens slightly, about 3 minutes. Turn off heat and slide saucepan to cool burner.
Carefully add vanilla and baking soda (mixture will bubble and foam). Add peanuts (if using) and stir to combine.
Ask an adult to use rubber spatula to carefully scrape caramel mixture onto popcorn in baking pan (saucepan will be heavy and caramel will be hot). Use rubber spatula to gently stir until popcorn is evenly coated. Spread popcorn into even layer.
Place baking pan in oven and bake for 20 minutes. Use oven mitts to remove baking pan from oven (ask an adult for help). Place baking pan on cooling rack. Use rubber spatula to carefully stir popcorn, scraping up caramel from bottom of pan (pan will be hot). Spread popcorn back into even layer.
Return baking pan to oven and bake until popcorn is deep golden brown, about 40 minutes, repeating stirring halfway through baking.
Use oven mitts to remove baking pan from oven (ask an adult for help). Place baking pan on cooling rack and carefully stir popcorn one last time (pan will be hot). Let caramel popcorn cool completely in pan, about 30 minutes. Break popcorn apart with your hands and serve. (Caramel popcorn can be stored at room temperature in airtight container for up to 5 days.)
Did you know that popcorn comes in different shapes? There are two main kinds: butterfly (also called snowflake) and mushroom. Most of the popcorn you get at the movies or buy as kernels in the supermarket is the butterfly shape, which pops up light and fluffy with lots of “wings” sticking out. Caramel popcorn, however, is often made with mushroom popcorn, which has a rounder shape (like a mushroom with a cap) and a more sturdy texture. Both popcorn shapes work for this recipe, but if you find mushroom popcorn in a specialty store or online, give it a try in caramel popcorn!