Compared to buttercream or canned frosting, Royal Icing has the benefit of hardening completely (and being fat-free). It's a must for gingerbread houses or intricately decorated holiday cookies. Find meringue powder at crafts stores.
When not using the icing, keep it covered with clear plastic wrap to prevent it from drying out.
Three Ways to Apply Royal Icing
Whichever method you choose, let the base coat dry before piping or painting on additional decoration.
Spoon: The simplest method is to spoon the icing over the cookie and spread it with the back of the spoon. This can be messy, but it doesn’t require any special tools.Brush: Spoon icing onto the cookie, but then spread it with an artist’s brush. This method affords some control without the pastry bags and two-step drying process of piping and flooding (below). Pipe and Flood: Using a pastry bag fitted with a narrow tip, pipe a rim of icing around the cookie. Let it set for an hour or so, then flood (fill) the center, using a brush, piping bag or squeeze bottle. Spread icing into tight corners with a toothpick. This method is fussier but offers the most precision.