Royal Icing was traditionally made with egg whites, but many people are fearful of salmonella nowadays. I don't worry as much about salmonella as I do about the raw egg whites spoiling when left at room temperature for long periods of time. So I use Meringue Powder royal icing.
Royal icing is a common icing for sugar cookies because it dries hard and can be thinned to have a smooth consistency. It isn't necessarily the best tasting icing, but for decorating it is the most versatile. It also dries quickly, which is a plus if you need to stack them.
5 Tablespoons Meringue Powder
3/4 Cup warm water
1 tsp cream of tartar
2 lbs Confectioner's Sugar, sifted
In a mixing bowl, combine the water and meringue powder together. Whisk by hand for 30 seconds until frothy. Add the cream of tartar and whisk again for 10 seconds, then add the sugar and attach the paddle attachment of your electric mixer. At medium-low speed (I use #2 on the KitchenAid) beat for TEN minutes until thick, creamy, and it forms stiff peaks. This stiff icing is the ideal consistency for constructing Gingerbread Houses. You can thin it by adding small amounts of water to achieve a flow consistency for cookies and such. Flow consistency should be when you drag a knife through the frosting and it takes a few seconds for the icing to smooth out and the knife mark to disappear.