Preheat oven to 200F. Line two cookie sheets with parchment paper.
Place the popcorn in a large stockpot – how we do it – or divide it between the two cookie sheets. Stir in the apple pieces and nuts, if using. (If using cookie sheets, divide the apples and nuts between the two before stirring well). Set aside.
In a large saucepan, melt the butter over medium heat. Stir in brown sugar, corn syrup, and salt, whisking until well combined. Bring to a boil over medium heat, stirring constantly. Boil for 5 minutes, remove from heat.
Stir in vanilla and cinnamon, whisking until well combined. (You don’t want clumps of cinnamon!). Add baking soda, stir well to combine – caramel will foam up a little.
Pour caramel over popcorn mixture, CAREFULLY stir to coat evenly – Hot sugar is nasty to get burned with! If you mixed it in a pot, divide the caramel corn between the two cookie sheets.
Bake for 1 hour, stirring well every 15 minutes. At the 30 minute mark, I like to switch pan positions in the oven – the one that was on the bottom goes on the top.
Once the hour is up, remove from the oven, break up large chunks of popcorn, and allow to cool. Serve immediately, or transfer to airtight containers for storage.