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, 1/2 teaspoon 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. Sprinkle with coarse salt if desired. Let cool completely before cutting into 12 bars.
Store in an airtight container, at room temperature, up to 2 days.