Begin melting the butter in a medium-large sauce-pan set over low heat.
Stir in the marshmallows.
Allow to melt, stirring occasionally, until about 75% of the marshmallows have melted.
Remove the pan from the heat and continue to stir, smashing the marshmallows with your rubber spatula or spoon, until melted and smooth.
Pour the cereal into a well-buttered large mixing bowl.
Pour the marshmallows over the cereal and stir until well combined.
Gently spread into a well-buttered 8-inch pan.
Allow the cereal treats to cool for at least 30 minutes before cutting into 16 squares.
If you prefer to use the microwave, heat the butter and marshmallows on high power for 1 minute. Remove and stir vigorously, smashing the marshmallows, until melted. If needed, heat for additional 15 second bursts of high power until melted. Then stir in the Fruity Pebbles Cereal and spread into an 8-inch pan.
Store in an airtight container at room temperature for up to 5 days.
Wrap well in non-stick tin foil or wax paper and place in a large zip-top bag to freeze for up to 3 months. Thaw at room temperature for at least 1 hour before opening the bag.