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 with a silicone spatula 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.