In a large bowl, combine cereal with the 1 additional cup of mini marshmallows. Set aside. Spray an 8x8 pan with cooking spray and set aside.
In a large pot, melt the butter over medium heat, stirring constantly. Once butter is melted, add brown sugar, whipping cream, and corn syrup. Cook over medium heat for 4-5 minutes, stirring constantly and scraping the bottom. Once caramel thickens, add salt and stir until combined.
Remove from heat and add the 10 oz bag of marshmallows. Stir until all marshmallows are melted and mixture is smooth.
Pour mixture over cereal and marshmallows. Mix until combined. Press mixture into the 8x8 pan. Cover and cool completely before cutting.