Pop the corn using an air popper or on the stove until you have about 4 quarts of popped popcorn. Transfer to a large bowl and remove any unpopped kernels.
In a medium saucepan, melt 1 cup of unsalted butter over medium heat. Stir in 2 cups of packed brown sugar, 1/2 cup of light corn syrup, and 1 teaspoon of salt. Bring to a boil while stirring constantly, then let it boil for 4-5 minutes without stirring.
Remove from heat and stir in 1 teaspoon of baking soda and 1 teaspoon of vanilla extract. Be cautious as the mixture will bubble up.
In the large bowl with the popped popcorn, add 2 cups of pretzel twists and 1 cup of roasted peanuts. Pour the warm caramel sauce over the mixture and gently stir to coat everything evenly.
Preheat the oven to 250°F. Spread the mixture evenly on two lined baking sheets and bake for 1 hour, stirring every 15 minutes.
Once baked, let the mixture cool completely before mixing in 1 cup of M&M's or holiday-themed chocolate candies.