Preheat the oven to 250Â°F. Line a rimmed baking sheet with parchment paper.
Pop the popcorn according to the package instructions. Coat a large mixing bowl with nonstick cooking spray, and transfer the popcorn from the bag to the bowl, taking care to pick out and discard any unpopped kernels.
In a medium saucepan, whisk together the brown sugar, corn syrup, butter, salt, and 2 tablespoons of water. Bring to a simmer over medium-high heat. Continue to simmer, whisking often, until the mixture reads 250Â°F on a candy thermometer, about 8 to 10 minutes. Immediately remove the pan from the heat, and whisk in the baking soda and vanilla. Quickly pour the hot caramel over the popcorn. Use a rubber spatula to gently fold the caramel into the popcorn, taking care to distribute it as evenly as you can. Stir in the peanuts, and transfer the mixture to the prepared baking sheet.
Bake for 1 hour, stirring and turning the popcorn with a spatula every 20 minutes. Remove from the oven, and place on a cooling rack for 20 minutes. Gently break up the popcorn, and serve.
Store in an airtight container for up to 5 days (or thereabouts).
Yield: about 10 cups