1. In a large mixing bowl, stir together powdered egg whites, cream of tartar, and confectioners' sugar. Stir in water. Using an electric mixer, beat on high speed for 5 to 7 minutes, until icing is smooth and stands up in stiff peaks. Stir in 2 tsp. lemon juice and beat on low speed until icing is smooth. Icing should be spreadable. If it seems dry, add more water, a few drops at a time, until smooth. If it seems runny, add more confectioners' sugar, 1 Tbsp. at a time.
2. If using icing for decoration, divide it among small bowls and tint with food coloring. If using it as glue for the gingerbread house, stir in an extra tablespoon or so of confectioners' sugar to stiffen.
3. Use immediately or cover icing tightly with plastic wrap and refrigerate. Icing hardens when exposed to air. Use within two days, as instructed.
Source: Hannaford fresh Magazine, November - December 2007