Over medium heat boil butter, brown sugar, salt and corn syrup for 5 minutes. Remove from heat and stir in baking soda. Pour over your popcorn and stir to coat evenly. Transfer to waxed paper lined counter and sprinkle nuts on top. Stir until all is coated evenly. Transfer popcorn into a large roasting pan and bake for 1 hour 15 minutes, stirring every 15 minutes.