Spray an 8x8 square cake pan with non-stick cooking spray and set aside.

In a very large, microwave-safe bowl, combine the marshmallows and butter. Microwave on high for 3 - 4 minutes. Remove and stir until butter and marshmallows are completely melted and combined.

Add the cereal and stir very well to coat the cereal completely.

Pour the cereal mixture into the pan and flatten with a spatula. Allow to set up... will take about an hour. The bars can be made a day ahead... just be sure to cover them.

Melt white chocolate in the microwave and stir until smooth. Should only take a couple of minutes. Spread evenly on top of the cereal bars and then top the white chocolate with sprinkles.

Slice into nine squares and serve.