Begin by popping the popcorn. You can use an air popper or a large pot on the stovetop. If using a pot, add a tablespoon of oil to the bottom, heat it over medium heat, and add the popcorn kernels. Cover with a lid and shake occasionally until the popping slows down. Once done, remove from heat and transfer the popcorn to a large mixing bowl.
In a heavy-bottomed saucepan over medium heat, melt the butter. Once melted, stir in the brown sugar, corn syrup, and salt. Bring the mixture to a boil, stirring constantly. Allow it to boil without stirring for about 4-5 minutes until it reaches a golden color.
Remove the saucepan from heat and stir in the vanilla extract and baking soda. Be careful, as the mixture will bubble up. Stir well until combined, then let it sit for a minute to cool slightly.
Pour the caramel mixture over the popped popcorn and peanuts. Use a wooden spoon or silicone spatula to gently toss everything together until the popcorn is evenly coated.
Preheat your oven to 250°F (120°C). Spread the caramel-coated popcorn mixture onto a baking sheet lined with parchment paper. Bake for 45 minutes, stirring every 15 minutes to ensure even coating and crisping.
Once baked, remove the popcorn from the oven and allow it to cool completely on the baking sheet. Once cool, break it into chunks and serve in a large bowl or store in an airtight container for later snacking.