Spray a 9x13-inch baking dish with non-stick cooking spray; set aside.
Toss together the popcorn, pretzels and peanuts in a large bowl; set aside.
In a medium saucepan, combine the sugar, water and 1 teaspoon of kosher salt; bring to a boil over medium-high heat. Boil, without stirring, until the mixture reaches an amber color, about 8 to 12 minutes. Remove from heat and slowly pour in the heavy cream (the mixture will bubble up and sputter). Immediately add the marshmallows and stir until melted.
Pour the salted caramel mixture over the popcorn, pretzel and peanut mixture and quickly stir together with a rubber spatula (I find that spraying the spatula with nonstick cooking spray makes this easier) until all of the popcorn, pretzels and peanuts are coated with the caramel mixture.
Pour the mixture into the prepared 9x13-inch pan. Spray a small piece of wax paper with nonstick cooking spray and use to press mixture evenly into pan. Sprinkle with some additional kosher. Cool completely and then cut into bars. These can be stored in an airtight container at room temperature.