In a large microwave-safe glass bowl, add the butter and microwave, uncovered, on HIGH 30 seconds to melt the butter.
Grease or coat with cooking spray a 13x9-inch baking dish or pan.
Add marshmallows to melted butter and stir well. Microwave, uncovered, 60 seconds. Stir well just until the marshmallows melt.
Add the rice cereal to the bowl of marshmallow mixture. Quickly stir just until all cereal is coated. Quickly spoon mixture into the greased dish and slightly press down the top of the mixture. You don't want to mash too hard so your bars will be light and fluffy. Let stand 20 minutes or until the bars are firm enough to cut.
In a medium-size microwave-safe glass bowl, add the white chocolate chips or candy melts. Microwave on HIGH 30 seconds. Stir well. They will not be completely melted. Microwave an additional 30 seconds. Stir until chips are melted. Microwave an additional 15 seconds, if needed.
Cut the cereal bars into 24 bars (6x4). Spread the melted white chocolate over the top of each bar. Let dry 10 minutes or until firm.