Preheat your oven to 215°. While the oven is heating, place the popcorn on a baking sheet and place it on the center rack of the oven.
Using a medium-size saucepan, melt the butter over medium heat. Add the sugar and corn syrup and stir until thoroughly combined. Whisk the mixture constantly until the butter and sugar come to a boil. Whisk in the baking soda, salt, vanilla extract, and cinnamon, and let boil for 1 more minute.
Take the popcorn out of the oven and drizzle the hot mixture over the popcorn then use a rubber spatula to gently fold until the popcorn is coated. It’s okay the popcorn isn’t completely covered. The oven will reheat the caramel and make it easier to coat the rest of the popcorn.
Place popcorn back into the oven and cook for 45 minutes, stirring every 15 minutes. Make sure to break up any large chunks before it cools completely. Remove from the oven and allow it to cool before serving.
Heating the popcorn before spreading the caramel sauce on it will make it easier to spread.
If you want to make chewy caramel corn, skip the oven part.
For a Cracker Jack taste, add a little molasses to the sauce and toss in some peanuts.