From Joy of Baking
Royal icing is wonderful. It dries quickly. It is smooth and matte and very hard. It can be used to decorate cookies or to stick walls of a gingerbread house together.
In the olden days, royal icing was made simply with raw egg whites and powdered sugar, with a little flavoring mixed in. But now, due to the risks of salmonella meringue powder is often substituted with very similar results.
Both recipes make about 3 cups of icing
Royal Icing using Egg Whites:
2 large egg whites (60 grams)
2 tsp lemon juice
3 cups (330 grams) confectioners sugar
This is perfect for items you will not be eating, like gingerbread houses. In a bowl, beat the egg whites for several minutes with the lemon juice mixed in. Then add the powdered sugar slowly, mixing until the icing is completely combined and smooth. Use immediately.
Royal Icing using Meringue powder:
4 cups confectioners sugar
3 Tbsp meringue powder
1/2 tsp flavor extract (lemon, vanilla, almond, etc...)
1/2 to 3/4 cup of warm water
Mix the confectioners sugar and meringue powder well. Add the water and beat until the mixture is very glossy and stiff peaks begin to form (about 5-7 minutes). You will know that your icing is the right consistency when you lift the beater, the ribbon of icing that falls back into the bowl remains on the surface of the icing for several seconds before disappearing.
It is important to keep this icing covered as much as possible, because it dries out very quickly. Feel free to add flavors and food coloring to your liking.