Line a 9x9-inch baking pan with parchment paper or foil. (You can spray the pan first with nonstick cooking spray to hold the lining in place, if needed.)
Measure out the cereal, then remove about ⅓ cup of the colorful marshmallow pieces. Set aside.
In a large pot, melt the butter over medium heat. Add the marshmallows. Stir frequently with a silicone spatula until fully melted, 5 to 7 minutes. Remove from the heat.
Immediately add the cereal and fold to combine. (It’s important to work quickly, as the marshmallows in the cereal will begin to melt.)
Pour the mixture into the baking pan. Use the silicone spatula or the butter stick wrapper, press the mixture into an even layer into the pan. Sprinkle the reserved marshmallows over the top and press them into the surface. Let cool completely, about 1 hour.
Use the parchment to remove the treats from the pan. Lightly grease a sharp knife and cut into squares.
Notes
Add a pinch of salt to the marshmallow and butter mixture to help balance the sweet flavors. Or sprinkle the top of the bars with flakey sea salt.
Work quickly. The marshmallows in the cereal will begin to dissolve when added to the hot marshmallows.
Spray the spatula with cooking spray when pressing the mixture into the prepared baking pan to prevent the cereal mixture from sticking.
Don’t pack the mixture too tightly. Press it into the pan, but don’t pack it too tightly, or the cereal bars will turn out hard and dense.
Soak the pot for easy cleanup. Melted marshmallows make a bit of a sticky mess, but it’s easy to clean up! Simply soak the pot in hot water with some dish soap to dissolve the marshmallows.