Icing, or the donut glaze, can't be rushed! Well it can be but then you might encounter lumpy icing and that's not what you want. Even topped with Lucky Charms they won't look so good. To make sure your icing comes out perfect, make sure you sift the powdered sugar to remove any lumps from it. Keep warming up the marshmallows and butter until the marshmallow are completely melted, before taking the pan off the heat. You might end up with some powdered sugar on the sides of the pan but you can use the milk to mix this in. If you are left with any little lumps you can always disguise them with some extra Lucky Charms!