Place 6 to 8 cups of popped popcorn in an extra large bowl. Line a jelly roll pan with parchment paper.
In a large saucepan, on medium heat, cook sugar, butter, and corn syrup. Stir regularly until it comes to a boil. Continue stirring until the caramel reaches 275-280 degrees on a candy thermometer.
Remove from heat. Add vanilla, salt, soda, and nuts and mix together quickly. The caramel will foam up as you stir in the ingredients.
Immediately pour the caramel over the popcorn, stirring to coat the popcorn before it hardens.
While still warm and pliable, transfer the caramel corn to the prepared pan and pull it into smaller sections to avoid large, heavy clumps. Let cool.
Each batch can be stored in a gallon-size storage bag.
Notes
No need to scrape the pan after pouring the caramel over the popcorn.I drizzle the caramel back and forth over the popcorn and then stir quickly to coat. You will notice the caramel changing to a light copper color right before it's reached the correct temperature.