In a medium saucepan, melt the butter over medium-low heat. Whisk in the brown sugar and maple syrup and continue stirring until mixture reaches a gentle simmer. Stop stirring and cook, undisturbed, for 5 minutes. Place popcorn in a large bowl. Line a sheet pan with parchment paper or foil.
Turn off heat and stir in the baking soda. The mixture will foam up briefly; stir gently until foaming mostly subsides.
Drizzle the caramel mixture (be careful, it’s really hot!) over the popcorn and gently fold, taking care not to crush the popcorn. Sprinkle the peanuts over the top and fold again. Carefully spread the mixture onto prepared sheet pan to cool, at least 10 minutes. Serve.