Pre-grease your pan and spatula with with a pad of butter or with non-stick cooking spray. *note: grease them liberally, or you will have marshmallows stuck everywhere.
Combine and Melt Marshmallows and Butter together. This can be done in small increments in the microwave or on low heat on the stovetop.
Make sure to stir throughout the melting time. If using the microwave, use 30 - 60 seconds at a time and stir in between heatings.
Add in the Fruity Pebbles cereal and then stir well to combine all ingredients.
Spread your mixture into a greased pan with a greased spatula. Making sure not to pack it down too much.
Let your treats cool for about 30 minutes, cut and then eat.