Measure out the marshmallows, butter, and cereal, and prepare the sprinkles in separate bowls for easy mixing.
Place the saucepan over low heat and add the butter. Stir until melted completely, and the butter begins to foam slightly with a fragrant aroma.
Add the marshmallows to the melted butter, stirring continuously until completely melted and smooth. The mixture should turn glossy and start to thicken slightly.
Remove the saucepan from heat. Gently fold in the puffed rice cereal and the colorful sprinkles using a spatula, ensuring even distribution and a vibrant mix. The mixture will become thick and sticky but spreadable.
Transfer the mixture into a greased or parchment-lined pan. Using the back of a spatula or your hands, press down evenly to compact the mixture into a uniform layer. Let it cool for about 20 minutes until set.
Once cooled and firm, cut into squares or rectangles. Serve immediately or store in an airtight container for later.