In a medium saucepan over low heat, melt the butter, stirring gently until fully melted to prevent browning.

Once the butter is melted, add the mini marshmallows and stir continuously until they are completely melted and blended with the butter, forming a smooth mixture.

Remove the saucepan from heat and pour the Fruity Pebbles cereal into the marshmallow mixture. Gently fold until evenly coated.

Line an 8x8 inch square baking dish with parchment paper. Pour the mixture into the pan, pressing it down evenly with a spatula.

Let the mixture cool at room temperature for about 30 minutes before cutting into squares or rectangles.