Line a square baking pan with parchment paper.
Place butter pieces in a large saucepan over low heat.
Melt butter over low heat just until fully liquefied, avoiding browning.
Add marshmallows and stir continuously.
Heat only until marshmallows are just melted and smooth, then remove from heat immediately.
Remove pan from heat.
Add puffed rice cereal gradually.
Stir gently until cereal is evenly coated.
Fold in part of the sprinkles for color.
Transfer mixture into prepared baking pan.
Press lightly into an even layer.
Sprinkle remaining festive sprinkles on top and let set before cutting.