12 cups plain popcorn (from 1/2 cup kernels or 2 microwave packages)
4 cups coarsely chopped small salted pretzels
2 cups sugar
1/2 teaspoon salt
2/3 cup heavy cream
2 cups miniature marshmallows
Coat a 9-by-13-inch baking dish with cooking spray. In a large bowl, toss together popcorn and pretzels. In a medium saucepan, bring sugar, salt, and 1/2 cup water to a boil over medium-high. Boil, undisturbed, until mixture is amber in color, 8 to 12 minutes. Remove pan from heat and slowly pour in cream (mixture will sputter). Immediately add marshmallows; stir until melted.
Pour caramel mixture over popcorn and pretzels and quickly stir with a rubber spatula to coat. Transfer mixture to dish and use a piece of plastic wrap coated with cooking spray to press evenly into dish.
Let cool completely before cutting into bars.