Pop the popcorn using two standard (3.5 oz) bags of plain microwave popcorn. Ensure the popcorn is unsalted and unbuttered. Once popped, measure approximately 7 quarts and divide evenly between two shallow, greased baking pans or roasting pans. Sprinkle the slivered almonds or other preferred nuts over the popcorn. Set aside.

Preheat the oven to 240°F (115°C). This low-temperature bake is essential for achieving crisp caramel coating.

In a medium saucepan, combine the brown sugar, corn syrup, butter, and salt. Stir the mixture over medium heat until the butter is melted and the ingredients are fully incorporated. Allow the mixture to come to a gentle, steady boil—do not rush this step by increasing the heat. Once the mixture begins boiling, continue to boil for 5 minutes, stirring constantly to prevent scorching.

Remove the saucepan from heat. Immediately stir in the baking soda and vanilla extract. The mixture will foam slightly—this is expected. Add a dash of cayenne pepper at this point if desired.

Carefully pour the hot caramel mixture over the popcorn and nuts. Using a spatula or large spoon, gently stir the popcorn to coat as evenly as possible. It is acceptable if the coating is not fully uniform at this stage.

Place both pans in the preheated oven. Bake for 1 hour, removing the pans every 15 minutes to stir the mixture. Stirring ensures even caramel distribution and prevents clumping.

Once baking is complete, transfer the caramel corn to waxed paper or parchment paper spread over a countertop. Allow it to cool completely. Once cooled, store in airtight containers. The caramel corn will remain fresh for up to 3 months when stored in a cool, dry place.