In a microwave-safe bowl, melt the butter over low heat. Once melted, add the mini marshmallows and stir until completely melted and smooth.

Remove from heat and quickly stir in the Fruity Pebbles cereal until evenly coated.

Transfer the mixture into a greased 9x9 inch baking dish. Press down firmly with a spatula to create an even layer.

Allow to cool for at least 30 minutes before cutting into squares.