Lightly spray a 9-by-9-by-2-inch baking pan with vegetable-oil cooking spray; set aside. Melt butter in a large pot over medium heat. Add salt and marshmallows; stir with a wooden spoon until melted. Remove from heat. Add Rice Krispies, Cap'n Crunch, and Froot Loops, and stir until combined.Advertisement
Transfer the mixture to the prepared pan. To prevent sticking, coat your hands with vegetable spray, and press the mixture evenly into the pan. Let cool, about 30 minutes.
Cut into 3-by-3-inch bars. Store in an airtight container for up to 3 days.