Line 2-1/2- to 3-qt. bowl with foil, extending foil about 2 inches beyond edge of bowl. Lightly grease foil; set aside.
Place butter in 4-qt. microwaveable bowl sprayed with cooking spray. Microwave on high 45 sec. or until butter is melted. Add marshmallows; stir to coat. Microwave 1-1/2 min. or until marshmallows are completely melted and mixture is well blended, stirring after 45 sec. Microwave an additional 4 min., stirring after 2 min. Add cereal; mix well.
Place cereal mixture in foil-lined bowl; press firmly onto bottom and up side of bowl to form thick shell. Let stand in cool dry place at least 4 hours or overnight until firm. Remove cereal shell from bowl, using foil handles. Carefully peel off foil. Fill shell as desired. Makes 24 servings.
According to Lucky Charms' Facebook, the unicorn marshmallow will begin showing up in cereal boxes across the US in March.
Unicorns have been a particularly trendy symbol in the culinary space over the last year. In 2017, Starbucks' color-changing Unicorn Frappuccino quickly went viral, bringing the sparkly and colorful style to the mainstream.