Combine the butter and mini marshmallows in a small glass bowl. The butter helps prevent the marshmallows from sticking and creates a smoother texture. Microwave the mixture for 10-15 seconds, watching carefully. The marshmallows will puff up and start to melt. Don't overcook or they'll become tough.

Stir the melted marshmallows well with a spoon. The mixture might clump into a ball, which is completely normal for this small amount. Add the puffed rice cereal to the marshmallow mixture. Spray your hands with nonstick cooking spray (this is crucial!) and mix everything by hand until the cereal is well-coated.

Press the sticky mixture into your prepared container, packing it down gently. Top with rainbow sprinkles if desired. Let the treat cool at room temperature for about 5 minutes, or enjoy it warm if you can't wait!