This is the traditional icing for glazing cookies, piping decorations, or assembling the walls of a gingerbread house. To make different colors, simply divide this big batch of icing into smaller amounts, then color each individually. Be sure to store any icing you're not using immediately in a tightly covered container; it becomes quite hard as it dries.
Place the meringue powder and salt in a large mixing bowl. Add the cool water, and stir slowly to allow the powder and salt to dissolve.
Allow to sit for 5 minutes for the powder to hydrate. After 5 minutes, rub a little between your fingers to feel for lumps or grittiness. If it's not smooth, let it sit for 5 more minutes before testing it again. Old meringue powder won't dissolve properly even when whipped with sugar, and your icing will be grainy and sandy.
Mix on low speed at first, to create a network of very fine bubbles. Add the sugar, increasing to medium and then high speed over several minutes. Beat until the icing is fluffy. You can adjust the sugar or water as needed to get a stiff, glossy icing that holds a tall peak.
Add your food coloring of choice, if using. Stir until the color is evenly incorporated.
Cover the bowl with a damp towel, and cover that with a layer of plastic wrap to keep the icing from crusting over.
To use for piping, put a tip in the bottom of a disposable pastry bag. Using a coupler will allow you to change tips with ease. Use a tall, heavy-bottomed glass to hold the pastry bag while you add the frosting. Take care not to fill the bag more than half full. Close the back of the pastry bag with a twist tie or spring clip, to keep the icing from backing up over your hand when you squeeze it.
Note that icing can take anywhere from 2 to 4 hours (and sometimes up to 24 hours) to set and dry fully once you've spread it on cookies or iced a cake, so be patient!
Royal icing is light, fluffy, and quite stiff: the more you beat it, the stiffer it becomes. For thinner icing, one that flows evenly over a cookie, simply dilute with a bit of water. Let icing dry to a hard, shiny surface, then pipe stiffer icing over it, or use food-safe markers to decorate.
Want to make royal icing using fresh egg white instead of meringue powder? Beat together 1 large, pasteurized egg white, 1 1/2 cups confectioners' sugar, 1/8 teaspoon salt, and 1 teaspoon lemon juice until stiff peaks form.
