Unlike a rudimentary powdered sugar and liquid (whole milk, water, or juice) glaze, which always dries translucent and is prone to flaking, royal icing has the right consistency for holding crisp lines and saturated color, and dries hard after a few hours. A bit of extract and salt make it tasty. Use as is for a thick icing that makes clear, defined outlines (use a piping bag with a variety of different piping tips for that), or thin with a little water to fill in your cookies with “flood icing.” Fresh egg whites work but using meringue powder cuts down the prep time (and eliminates leftover egg yolks) and makes the smoothest icing that is virtually foolproof.