Spray an 8-inch square cake pan with nonstick spray.
Add the cereal to a large heatproof bowl and set aside.
Add the butter to a medium saucepan over medium-high heat. Once melted, 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. Turn off the heat and stir in the marshmallows. Keep stirring until the marshmallows are smooth and melted.
Quickly pour the caramel marshmallow sauce over the cereal and stir all together. Pour into the prepared pan and press down with a piece of parchment paper sprayed with nonstick spray. Sprinkle lightly with flaky sea salt.