Pre-heat oven to 250 degrees.
Place popcorn and nuts (if using) in a large bowl, set aside.
Add corn syrup and brown sugar to a small bowl.
Chop butter into small chunks and add to bowl with corn syrup and sugar. Microwave for 30 seconds and then stir.
Microwave for 2 more minutes and stir.
Return to microwave for another 2 minutes. Stir to combine. The mixture should be hot and bubbly.
Add in vanilla, baking soda, and cinnamon (if using). The mixture will get a bit foamy.
Stir and then pour over the large bowl of popcorn. Mix well to coat popcorn.
If you want your caramel corn very chewy, go ahead and eat it now. Otherwise, pour caramel corn onto a foil or parchment paper lined baking sheet in a single layer.
Bake for 5 minutes, stir and taste. Keep baking and stirring every 5 minutes (up to 20 minutes of baking) until you get it as crunchy as you want. It will harden up a bit as it cools. Break apart and devour!