Make ahead: The icing can prepared up to 2 days ahead, stored in an airtight container with a moist paper towel pressed directly on the icing surface, and refrigerated.
This icing hardens into shiny white lines, and is used for piping decorations on gingerbread people or other cookies. Traditional royal icing uses raw egg whites, but I prefer dried egg-white powder, available at most supermarkets, to avoid any concern about uncooked egg whites.
When using a pastry bag, practice your decorating skills before you ice the cookies. Just do a few trial runs to get the feel of the icing and the bag, piping the icing onto aluminum foil or wax paper. If you work quickly, you can use a metal spatula to scrape the test icing back into the batch.
Dried egg-white powder is also available by mail order from The Baker's Catalogue, 1-800-827-6836. Meringue powder, which is dehydrated egg whites with sugar already added, also makes excellent royal icing; just follow the directions on the package. However, the plain unsweetened dried egg whites are more versatile, as they can be used in savory dishes, too. Meringue powder is available from Adventures in Cooking (1-800-305-1114) and The Baker's Catalogue.
In a medium bowl, using a hand-held electric mixer at low speed, beat the confectioners' sugar, egg-white powder and water until combined. Increase the speed to high and beat, scraping down the sides of the bowl often, until very stiff, shiny and thick enough to pipe; 3 to 5 minutes. (The icing can be prepared up to 2 days ahead, stored in an airtight container with a moist paper towel pressed directly on the icing surface, and refrigerated.)
To pipe line decorations, use a pastry bag fitted with a tube with a small writing tip about 1/8-inch wide, such as Ateco No. 7; it may be too difficult to squeeze the icing out of smaller tips. If necessary, thin the icing with a little warm water. To fill the pastry bag, fit it with the tube. Fold the top of the bag back to form a cuff and hold it in one hand. (Or, place the bag in a tall glass and fold the top back to form a cuff.) Using a rubber spatula, scoop the icing into the bag. Unfold the cuff and twist the top of the bag closed. Squeeze the icing down to fill the tube. Always practice first on a sheet of wax paper or aluminum foil to check the flow and consistency of the icing.
Traditional Royal Icing: Substitute 3 large egg whites for the powder and water.