Prepare 8x8 glass or metal dish by spraying liberally with nonstick cooking spray. Set aside.

In a large bowl, add 4 cups Lucky Charms cereal. Separate out Lucky Charms marshmallows (about 1/2 cup) and set aside.

In microwave-safe bowl, microwave butter and 3 cups mini marshmallows on High for 2 minutes.

Stir marshmallow mixture until smooth. If marshmallows are not fully melted, microwave additional 30 seconds and stir again.

Pour melted marshmallow mixture onto Lucky Charms cereal and stir well with silicone spatula, making sure all cereal is evenly coated.

Using wax paper, press marshmallow-coated Lucky Charms into prepared 8x8 dish.

In same bowl that marshmallows were melted in, microwave remaining 1/2 cup of mini marshmallows on High 30 seconds. Stir and spread melted marshmellow mixture over top of Lucky Charms treats.

Press Lucky Charms marshmallows into top of treats and sprinkle with green sugar sprinkles as desired.

Allow to set 30 minutes. Cut into 12 bars. Best if served the same day.