Preheat the oven to 250℉. Line a baking sheet with parchment paper.
In a saucepan over medium heat, combine the butter, brown sugar, salt, and corn syrup. Bring the mixture to a boil and boil for 3 minutes, whisking constantly. Remove from the heat and immediately whisk in the baking soda, spices, and vanilla.
Immediately pour over the popcorn in a large mixing bowl. Working quickly, stir the popcorn using a spatula until evenly coated with the caramel mixture. Fold in the nuts, if using. Spread into an even layer on the prepared baking sheet.
Bake in the preheated oven for 30-35 minutes, stirring every 15 minutes (this avoids ending up with one giant blob of caramel corn and helps it get crunchy). Remove from the oven and let cool down on the baking sheet. The caramel corn will harden and crisp up as it cools. Enjoy!
Notes
*Unsalted may be used instead with an additional ¼ teaspoon of salt.**White sugar plus 2 teaspoons of molasses may be used in place of brown sugar.***I often use Roger's Golden Cane Syrup in place of corn syrup.****1 ½ teaspoons of a pumpkin pie spice blend may be used in place of all individual spices.Store caramel corn in an airtight container at room temperature.