A blend of egg whites and confectioners’ sugar, royal icing hardens to a durable, rock-hard consistency when allowed to dry. Start by making a batch of Royal Icing and then coloring it however you like. You can split the batch up and make lots of colors, or you can leave it white.
To outline a cookie
Spoon some of the icing into a pastry bag fitted with a very small plain tip and outline the rim of a cookie with the icing. (Practice first on a piece of cardboard or waxed paper. If the icing is too thick to pipe evenly, put it back in the bowl and stir in water, a drop or two at a time, until it pipes easily but still retains its shape.) Scatter sprinkles or sparkling sugar over the icing, if you like. Set the iced cookies aside to dry.
To coat an entire cookie with icing
Have ready a small clean artist’s brush (one that you use only for food). If you want to use colors, set out a bowl for each color, portion the icing into the bowls, and stir drops of food coloring into each until the desired shade is reached. Outline the rim of a cookie with the icing as described above and let harden slightly. Dampen the brush in water and spread a small amount of additional icing in an even layer within the cookie’s border. Decorate the cookie with sprinkles, sparkling sugar, or edible dragées, if you like. Set the cookie aside to dry.
Once the icing is completely dry and hard, store the cookies in airtight containers at room temperature for two to three days or in the freezer for longer storage.