Heat oven to 250ºF.
Grease a sheet pan with nonstick spray and line with parchment paper. Spray the parchment paper with nonstick spray as well and set aside.
Combine the popcorn and peanuts in a large mixing bowl. Set aside until ready to use.
Melt the butter in a medium saucepan over medium heat. Add the brown sugar, corn syrup, and vanilla, and stir until combined. Heat the mixture until it reaches 250ºF on a candy/fry thermometer, approximately 10 minutes.
Pour the syrup over the popcorn and peanuts and stir to combine. You will need to work quickly because the syrup hardens rapidly. Spread the mixture onto the prepared sheet pan and bake in the oven for 1 hour. Cool completely.
Break into pieces and serve immediately or store in an airtight container for 2 to 3 days.