Preheat oven to 250 degrees.
Place popcorn and peanuts (if using) in a large bowl and set aside.
Add the brown sugar, corn syrup, and butter to a small microwave safe bowl.
Microwave for 2 more minutes and stir.
Return to microwave for another 2 minutes. Stir to combine. The mixture should be hot and bubbly.
Stir in the 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 and sprinkle lightly with salt.
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 and then stir and taste. Keep baking and stirring every 5 minutes (up to 20 minutes of baking) until the popcorn is as crunchy as you'd like. It will harden up a bit as it cools.
Break apart any large clumps and let cool.