Heat the oven to 250°F. Line a rimmed sheet pan with parchment paper and spray it with cooking spray. Add the popcorn and peanuts to a large bowl.
Add the brown sugar, butter, corn syrup, and salt to a medium saucepan with high sides. Cook over medium heat, stirring constantly, until the mixture comes to a boil.
Once it's boiling, set a timer for 2 minutes and keep stirring.
Remove the pan from the heat. Stir in the baking soda and vanilla. The caramel will bubble up a lot, so be careful.
Pour the caramel over the popcorn and peanuts right away. Be careful, this is VERY HOT. Stir quickly until the popcorn is coated. A few uncoated pieces are fine.
Spread the caramel corn onto the prepared sheet pan in a single layer. Bake for 15 minutes.
Stir, spread it back into a single layer, and bake 15 minutes more. It should look a little glossy but dry, with caramel coating each piece of popcorn.
Let the caramel corn cool at room temperature. It will be soft while it's hot but will crisp up when it's cooled. Break it into chunks and store in an airtight container.