Preheat the oven to 250°F. Spray two large deep roasting pans with butter cooking spray.
Divide the popcorn between the baking pans and sprinkle the peanuts on top. I recommend party or cocktail peanuts and not the dry roasted for this popcorn. Remove any loose unpopped kernels.
In a heavy bottomed saucepan melt together the butter, brown sugar, corn syrup and salt.
Stir constantly until the mixture comes to a boil then lower the heat. Continue to boil for 5 minutes. Set a timer for this step. There's no need to stir it constantly, however you should give it a good stir periodically to keep the caramel from sticking to the bottom.
After 5 minutes remove from the heat and add the baking soda stirring constantly. The baking soda will cause the mixture to foam up continue to stir until the foam begins to die down. Add 1 1/2 tsp vanilla, stir well.
Divide the mixture between the two pans of popcorn and stir until coated. The popcorn won't all be perfectly coated at this stage.
Place the pans into the oven and bake for 1 hour stirring at 15 minute intervals.
Spread the popcorn onto wax paper to cool. Store tightly covered at room temperature.