Place the cereal in a large heatproof bowl and set aside.
Melt the butter in a medium saucepan over medium-high heat. Stir in the brown sugar, heavy whipping cream and corn syrup. Cook until thick and syrupy while stirring frequently, about 7 minutes. Stir in the salt. Remove pan from the heat and stir in the marshmallows. Continue stirring until the marshmallows are smooth and melted.
Pour the caramel marshmallow mixture over the cereal and stir together. Scrape into the prepared pan and press down with a piece of parchment paper sprayed with nonstick spray. Sprinkle lightly with salt.